Addressing sedimentation and pollution

Coastal land development has had a significant impact on the quality of coastal waters through increasing sediment and pollutants flowing into the ocean. Early Māori burnt areas of coastal forest and established agriculture around coastal plains. Sediment cores demonstrate that this caused increased amounts of silt and mud to accumulate in estuaries and harbours. Further vegetation clearance, significant changes in land use and rapid modification of coastlines following European settlement, have exponentially increased soil erosion and sediment accumulation rates in many New Zealand estuaries.

In the mid-1900s a large proportion of sewage was being discharged untreated into the sea or waterways in New Zealand. Most of what was treated, only received primary treatment, which simply involved removing solids before discharge. By the 1970s untreated discharges had almost halved. In 1991 the RMA was enacted resulting in tighter controls on point source discharges, but diffuse discharges still remain a challenge. 3146  There is likely to be continued deterioration of coastal water quality, even without new development, if remedial action is not taken. This is because the legacy of past activities has resulted in the accumulation of pollutants and nutrients in soils. Sediment often still contains materials now banned, such as DDT, tin from antifouling and lead from petrol. Coastal works that involve disturbing these sediments can redistribute the pollutants.


In recent years sedimentation, in particular, and pollution have been recognised as a major concern for the coastal environment and its biodiversity. Degradation of water quality, and the subsequent impact on coastal ecosystems, has resulted from both rural and urban point and non-point sources. The vision of the NZCPS 2010, as set out in Objective 1, is to maintain coastal water quality and enhance it where it has deteriorated as a result of human activity.

Achieving this will require baseline analysis, an understanding of any trends in water quality through good monitoring, and knowledge of the activities that impact on them. In addition, there is now a clear expectation that action will be taken to minimise the discharge of sediment and pollutants from diffuse sources on land and marine-based activities. Councils will need to ensure that the background information used to inform their plan provisions, recognises that water bodies are interlinked, and that coastal water quality is often determined by activities and discharges further up the catchment.


Land and marine-based activities can result in a number of key impacts on the coastal environment:

  • Vegetation clearance (including that from forestry harvesting), overgrazing, intensive pastoral land uses, poor riparian management and earthworks (both urban and rural) can increase the amount of sediment and other contaminants flowing into the ocean
  • Increasing the amount of land covered with hard surfaces can result in less infiltration and greater runoff within a catchment. This, in turn, can lead to greater erosion of stream channels and a consequent increase in the amount of sediment discharged into the sea
  • Urban infrastructure, including stormwater and sewage reticulation systems, will often involve discharges to the coastal marine area. These discharges can be particularly challenging to manage in extreme rainfall or other emergency situations
  • Many contaminants adhere to sediments, particularly the finer particles. Through a combination of physical factors (discharging into calm settling zones) and chemical processes (higher salinity causing fine particles to flocculate and settle) there is an accumulation of contaminated sediment in estuaries
  • Marine works, such as dredging and dumping, can result in the re-suspension of sediment from the sea floor

Suspended sediment is thought to be the main cause of reduced water clarity in coastal areas. The discharge of sediment into the marine area can have profound effects on marine ecosystems through a number of changes, including:

  • Altering habitats on the sea floor
  • Smothering bottom-living organisms
  • Clogging the gills of filter feeders
  • Reducing the clarity of the water
  • Poisoning marine life where sediments carry toxins

Discharges of sewage and polluted stormwater increase the nutrients and pollutants in seawater and marine sediments. Sewage is a significant source of nutrients flowing into many harbours and coastal areas adjacent to settlements through:

  • Inadequate treatment of sewage before it is discharged into the marine area
  • Leakages and overflows from sewerage treatment systems spilling into stormwater drains and ultimately into the sea; these events will often increase in extreme rainfall events
  • The failure of irrigation systems from on-site sewerage treatment — often those connected to septic tanks

High levels of fertiliser use on rural land, and intensive farming, can also result in elevated levels of nutrients entering the marine area. This occurs particularly with dairy farming, but also with intensive (irrigated) cropping, horticulture and sheep and beef farming. High nutrient levels can result in the rampant growth of nuisance seaweeds and bottom algal growths which smother other marine life, and can exacerbate blooms of suspended algae. Algal blooms can reduce water clarity and in some cases can produce toxins which variously kill fish, render the water unfit for human contact, make shellfish toxic and even affect people on land when there is an onshore wind.

Development which significantly increases road traffic can have negative effects on the marine environment. This is especially the case in the Auckland region where traffic volumes are particularly heavy. Many heavy metals accumulating in marine sediments in the upper Waitematā harbour are derived from motor vehicles. Lead, zinc, copper and polycyclic aromatic hydrocarbons originating from the vehicles and their fuel, become deposited on roads, and then wash into waterways.

Runoff from unpainted iron roofs can also be a major source of zinc in marine areas. Boatyards and boat antifouling can generate pollutants and toxic substances which end up in coastal waters, as can oily discharges from boats. There are regional differences throughout the country in the contaminants found in the marine area and historic contamination is a major source of heavy metals in some locations.

New Zealand Coastal Policy Statement

Most of the issues associated with degradation of marine water quality are the result of land use and discharges within catchments, meaning that coastal water quality is primarily managed through provisions in regional plans applied above mean high water springs and through the resource consent process. The RMA provides that no person may discharge any contaminant into water unless the discharge is expressly allowed by a rule in a regional plan or by a resource consent. 3147  A regional council can only allow such a discharge, as a permitted activity in a regional plan, if it is satisfied that the discharge will not have any significant adverse effects on aquatic life, amongst other things.

Although these provisions enable good control to be exercised over point-source discharges into the sea, it is more difficult to address diffuse non-point discharges, where contaminants and sediment wash off the land and end up in the marine area. In some cases it is possible to reduce the generation of contaminants at their source, through adopting low impact design, or painting galvanised roofs to minimise discharges of zinc. In other cases discharges require controls over earthworks, land clearance and land management activities. This may include the adoption of treatment techniques and stormwater detention ponds. There are challenges in obtaining good information on which to base management responses, for example, through quantifying contributions from different sources and understanding how contaminants behave in the coastal marine area. Hydrodynamic modelling may be required and can be costly.

Enhancement of water quality

Policy 21 of the NZCPS 2010 addresses enhancement of water quality in the coastal environment and includes the following directions to regional councils:

  • “Priority” is to be given to improving the quality of water where it is having a significant adverse effect on ecosystems, natural habitats, or waterbased recreational activities, or is restricting existing uses such as shellfish gathering
  • Such areas of coastal water and water bodies are to be identified and included in regional plans
  • Provisions should be included in regional plans to improve water quality in the areas identified above. Water quality should be restored at least to a state that can support activities, ecosystems and natural habitats, where practicable
  • Stock must be excluded from the identified areas, including from the coastal marine area, adjoining intertidal areas and other water bodies and riparian margins in the coastal environment, within a prescribed time frame

The Australian and New Zealand guidelines for fresh and marine water quality promulgate a water quality management framework that could be useful for the purpose of implementing Policy 21. These guidelines provide an important reference for water quality management in New Zealand, particularly in regard to toxic contaminants. They offer methods for setting limits on pollutant concentrations in freshwater, coastal and marine environments. 3148

Managing sedimentation

The effects of sediment discharges (such as turbidity and sedimentation), either as a result of land-based or coastal activities, have been identified as one of the most serious threats to the integrity of New Zealand’s estuarine and coastal ecosystems. 3149

Policy 22 of the NZCPS 2010 addresses sedimentation in the coastal environment. Activities will not need to actually be in the coastal environment to be potentially relevant to the implementation of Policy 22. For example, a major inland roading project involving large-scale earthworks may be of concern where waterways lead to the coastal marine area.

Policy 22 requires councils to:

  • Assess the current levels of sedimentation in coastal waterways and the marine area
  • Identify the likely environmental impacts of current sedimentation levels
  • Monitor changes in sedimentation levels and impacts
  • Ensure that there is no significant increase in sedimentation in coastal waterways and the marine area
  • Control the impacts of forestry harvesting on sedimentation as well as other vegetation removal
  • Control land use activities to reduce sediment loadings in runoff and stormwater systems

Reducing the discharge of contaminants

The discharge of contaminants into the coastal marine environment is a key issue for addressing coastal pollution and enhancing coastal water quality. Policy 14 recognises that in order to restore the natural character of the coastal environment there is a need to reduce or eliminate discharges of contaminants.

Policy 23 of the NZCPS 2010 deals with this topic directly and includes a comprehensive list of matters that councils must have regard to. This particular policy arose from the recommendation of the Board of Inquiry that water quality policies be strengthened. This was to ensure that decision-makers who are considering discharges to water in the coastal environment have proper regard to how to prevent or minimise adverse effects, particularly on ecosystems, habitats and the lifesupporting capacity of water. 3150

Policy 23 provides clear direction that discharges of untreated human sewage into coastal water should not be occurring. In addition, the discharge of treated sewage into the coastal environment should only be permitted where there has been adequate consideration of alternatives and the effects on tangata whenua values have been understood and considered. There must also be early and meaningful consultation with tangata whenua when developing plan provisions which provide for such activity. Policy 23(4) sets out what local authorities need to be doing to effectively manage stormwater discharges.

This includes:

  • Adopting an integrated catchment management approach which links the management of stormwater networks with that of other activities within the catchment
  • Reducing contaminant and sediment loadings at their source, through such actions as placing controls on land use activities, adopting good design, and requiring stormwater treatment
  • Addressing cross contamination between stormwater and sewage systems

The Department of Conservation has prepared some brief guidance on implementing Policy 23. This makes it clear that when considering the discharge of treated sewerage into the coastal environment, councils will need to consider the location, frequency, duration, volume, level of treatment and the extent to which the discharge is the best practicable option (within a relevant timeframe). 3151  The guidance also states that Policy 23 needs to be considered together with objectives and other policies in the NZCPS 2010 which address issues of discharge, water quality and infrastructure, giving a more holistic approach to addressing this issue.

Best Practice Planning Elements

Regional and district plans can include the following in order to address sedimentation and pollution:

(1) Undertake a wide assessment of sedimentation

The management of sedimentation cannot be left to individual resource consents as it is difficult, if not impossible, to deal with cumulative effects on a case-by-case basis.

  • Review sedimentation issues from “top-of-catchment to harbour” analysing sediment yield from all sources including stream banks, coastal erosion, stormwater discharges and individual earthwork sites
  • Project likely sediment yields based on future urban development
  • Assess likely effects of current and projected future sediment levels on coastal ecosystems

Example: Mahurangi Action Plan, Auckland

Sediment is the priority issue in the Mahurangi catchment. Rates of sediment entering the Mahurangi harbour have significantly increased as a result of human activities, including deforestation, development and intensive land use. 3152

Actions identified to reduce sediment generation include:

  • Developing a riparian management programme which includes retiring land, fencing and replanting of riparian and foreshore corridors, prioritising areas for action and providing funding
  • Planting shoreline and stream margins and fencing where adjoining land will be grazed
  • Educating landowners on best practice methods of development and land use (such as for roading, farming and forestry) and simple stormwater solutions such as adopting green engineering and low impact design to reduce the impact of stormwater from heavy rainfall and storm events
  • Balancing economic return and environmental effects of land use
  • Promoting engagement with land user groups to promote less sediment generating forms of land use

(2) Control land-based activities that can generate increased levels of sediment and pollution

Set objectives, policies and rules in regional policy statements, regional plans and district plans, and through conditions attached to resource consents, that control a range of activities such as earthworks, vegetation clearance, forestry, agriculture and stock grazing in the coastal environment to minimise levels of sedimentation and pollution.

  • Consider whether current planning controls for earthworks and other sediment-generating activities such as forestry harvesting and track construction are adequate
  • Consider whether there are particular erosion-sensitive sites in a catchment which need more careful management and rigorous controls
  • Provide for generous setbacks from coastal margins for coastal development, including riparian margins
  • Require the retention of riparian buffer zones along waterways and the coastal edge for farming and forestry activities to prevent large amounts of sediment from entering the coastal marine area
  • Develop and implement controls to minimise the discharge of nutrients from rural land uses

Example: Christchurch City Plan Chapter 10 Water Resources

Objective 1: To assist in the protection of the availability of water and in the preservation and enhancement of water quality of surrounding coastal waters and the underground and surface fresh water of the District.

Policy 1A: To ensure that activities do not adversely affect the quality of surface, ground and coastal waters and their associated aquatic habitats.

Policy 1C: To promote measures that minimise non-point source discharges of contaminants into water bodies and the coastal area.

Policy 1D: To promote the use of land to maintain and where possible enhance water quality.

Policy 1E: To retain, protect and promote the establishment of riparian vegetation.

(3) Control generation and disposal of stormwater and sewage

Set objectives, policies and rules that control on-site stormwater and sewerage generation and disposal. Further advice and guidance can be found in Auckland Council’s Technical Publications (including TP10 – Design guideline manual stormwater treatment devices, TP58 (third edition) – On-site wastewater systems and TP124 – Low Impact Design Manual for the Auckland Region).

  • Include site coverage limits
  • Encourage use of porous surfaces in developments
  • Encourage use of green infrastructure including constructed wetlands and swales to manage stormwater
  • Include density limits
  • Provide for generous setbacks from coastal margins for coastal development, including riparian margins
  • Consider management options in extreme weather events, equipment failures and emergency events

Example: Horizons Regional Council manual for On-site Wastewater Systems design and management

Permitted Activity Rules 1.3.3 New and upgraded discharges of domestic wastewater.

The discharge of domestic wastewater into or onto land from an on-site wastewater treatment and disposal system which either—

(1) Is newly established after this rule becomes operative; or
(2) Involves the upgrade of a system that existed prior to this rule coming into effect; is permitted provided: …
      (c) Where the property within which the discharge occurs is less than 10 ha
              (i) The property shall cover an area of at least either 5000m2 for properties created by subdivision after this rule comes into effect, or 2500 m2 for properties that existed prior to this rule coming into effect.

(4) Control generation of sediment and pollution from marine-based activities

Set objectives, policies and rules that control the location of marine-based activities that have the potential to increase sedimentation and pollution of the coastal environment.

  • Ensure contaminated discharges from marinas and boat haul out yards are captured and prevented from entering the wider marine area
  • Encourage the location and design of marine facilities to reduce the need for dredging
  • Identify the most appropriate disposal location for dredged material from an environmental, social, cultural and economic perspective. This will depend on the circumstances, particularly the quantity and quality of the sediment to be disposed of and the availability of suitable land-based disposal sites
  • Control stocking rate and feed levels associated with fed aquaculture

Example Regional Coastal Plan for Northland – Chapter 22 Dredging and Dredging Spoil Disposal

Objective 22.3: Provision for capital and maintenance dredging that is needed for the establishment and operation of appropriate facilities in the coastal marine area (such as Marinas and Ports), while avoiding, remedying, or mitigating the adverse effects of such dredging and any associated spoil disposal in the coastal marine area.

Policy 22.4.7: To promote land-based disposal of dredging spoil from both capital and maintenance dredging of the coastal marine area, where this better meets the purpose of the Act.

Explanation: Disposal of dredging spoil to sea or into intertidal areas can create significant adverse effects. In most situations, spoil disposal to land avoids these effects and therefore should be used where practicable.

Method 22.5 .9: In processing applications for resource consents for dredging spoil disposal, require evaluation of the costs and benefits of land-based disposal options.


(5) Support the use of other methods and tools

Incorporate a range of other tools and methods to address sedimentation and pollution.

  • Provide incentives for the revegetation of land and other remedial measures
  • Establish high standards for the provision of sewerage and stormwater infrastructure which is constructed by the developer and transferred to the council on completion through Long Term Plans and Annual Plans (prepared under the Local Government Act 2002)
  • Prevent the expansion of water reticulation into areas where there are existing septic tank soakage problems. The availability of reticulated water increases water use and exacerbates the soakage problem
  • Retrofit, renew and replace stormwater and sewerage infrastructure which is contributing to water pollution. New development can add to an existing sewer overflow problem and can be the trigger for the provision of community sewerage systems, particularly in areas where septic tanks have proven inadequate
  • Provide alternative hazardous wastes disposal options for residents and industry

Example: Bay of Plenty Regional Land and Water Plan (2008)

4. Discharges to water and land

4.2 Discharge of stormwater

Method 120: Encourage measures to reduce the volume of stormwater discharged to the environment from urban areas, including:

(a) The appropriate design of subdivisions and other land use developments to minimise stormwater runoff, such as minimising the increase in the area of impermeable surfaces and retaining natural flood retention areas.
(b) On-site management and disposal of stormwater to soakage, where practicable and appropriate.
(c) Storage and reuse of stormwater, including for irrigation or creation of aquatic habitats, where practicable and appropriate.
(d) Retention or creation of non-structural stormwater controls, where appropriate…

Method 123: Encourage district and city councils to investigate measures to return piped streams in urban areas to their natural state, where practicable and achievable within existing urban areas.

Best Practice Design Elements

The level of attention to sedimentation and other water-borne pollutants should be influenced by the significance of likely effects, including consideration of such matters as the scale of the project and sensitivity of the receiving environment. Development designed to address sedimentation and pollution can include the following elements:

(1) Incorporate sediment retention mechanisms into earthworks

Undertake earthworks in the dry season and incorporate sediment generation limitation and retention mechanisms:

  • Minimise earthworks and disruption to vegetation cover and soils
  • Work within the natural contours of the site rather than modifying it
  • If possible, carry out earthworks during the summer
  • Avoid erosion by diverting surface water away by using runoff diversion channels, contour drains or earth bunds
  • Put in place silt control measures such as silt fences, hay bales, vegetation buffer strips, sediment ponds and earth bunds
  • Include a generous riparian buffer around waterways
  • Avoid works which involved the disturbance of sediments known to be contaminated
  • For large projects, stage works to reduce soil exposure at any one time

(2) Forward planning and use of cutting edge technologies

There are a range of treatment devices that can be used, including swales, filters and infiltration devices such as rain gardens. Further advice and guidance can be found in the Auckland Council’s Technical Publication TP10 – Design guideline manual stormwater treatment devices.

  • Use on-site sewerage treatment and disposal systems that reduce nutrients
  • Locate on adequate lot sizes to allow proper infiltration
  • A “treatment train” approach incorporating one or many treatment devices in a series can provide an optimal solution. This approach depends on the nature of the contaminant to be removed and site constraints, such as available land
  • Use low-flow, water-efficient appliances and fittings to minimise water use and sewage generation. This is particularly important where land disposal is proposed
  • Incorporate community sewerage schemes which are designed to high standards with overflows and treatment options

Desirable example: Wynyard Quarter, Auckland Waterfront

Wetlands and indigenous plantings have been created to assist with the filtration of stormwater in a highly modified environment.

Auckland Waterfront Wetlands (Credit: Raewyn Peart)

(3) Reduce runoff from urban activities

Design developments to ensure hard surfaces are reduced and hydraulic neutrality is provided (so that the increase in stormwater runoff due to hard areas, such as pavements and buildings, does not exceed the rate of runoff). For buildings and structures, include the provision of riparian margins and generous setbacks from the coastal edge.

Locate buildings well back from the foreshore edges

  • Maintain existing natural vegetation and landform buffers
  • Incorporate planting along riparian margins
  • Minimise hard surfaces and increase green areas
  • Ensure no more runoff and no faster than in the prior conditions
  • Use attenuating water tanks
  • Use natural soakage and drainage patterns
  • Use newly-created wetlands rather than existing wetlands for water treatment
  • Use swales and green areas rather than hard channelling
  • Incorporate rain gardens
  • Include mid-catchment ponds

Undesirable example: Leith River, Dunedin

Hard channelling and lack of riparian planting increases sediment and pollutants reaching the marine environment.

Leith River, Dunedin (Credit: Raewyn Peart)


Desirable example: Project Twin Streams, Auckland

By providing a system of riparian areas and plantings, urban stormwater can be more effectively filtered before it reaches the coast.

Project Twin Streams (Credit: Project Twin Streams 2015)

(4) Reduce runoff from agricultural activities

Incorporate mechanisms that reduce the runoff produced by agricultural activities.

  • Prevent stock from accessing waterways and the coastal edge
  • Have adequate farm treatment systems to cope with the volume and strength of effluent produced and discourage overstocking
  • Create riparian buffer zones using native flora
  • Construct runoff management systems such as wetlands or pools along subsurface farm drains
  • Restore old wetlands to create natural obstacles to sediment runoff
  • Construct farm tracks on sites where gradients are low
  • Optimise fertiliser application such that nutrient discharges are minimised
  • Develop Farm Management Plans to minimise nutrient discharge with practices such as optimising stocking rates, moving stock to “winter runoffs” to “spell” pasture in more nutrient discharge prone areas, or using feed pads with effluent treatment facilities.

Desirable example: Manawatu-Wanganui Guide to Managing Farm Dairy Effluent

The council has published a useful guide which offers advice and information on ways to reduce runoff from agricultural activities.

(5) Reduce runoff from forestry activities

Incorporate mechanisms that reduce the runoff produced by forestry activities:

  • Undertake tree felling and track construction in plantation forests so that a riparian buffer and coastal margin is retained to protect the catchment from sedimentation
  • Install cut-off drains at regular intervals on tracks so that water runoff can be managed and directed away from waterways
  • Avoid felling and track construction during high rainfall periods
  • Use sediment control measures when harvesting trees on steep or erodible slopes
  • Maintain riparian buffer zones along waterways and coastal edges to prevent large amounts of sediment from entering the coastal area
  • Avoid highly invasive mechanical and burning techniques to clear land with high erosion potential

Desirable example: Auckland Council Technical Publications

Auckland Council’s Technical Publication TP90 – Erosion and sediment control guidelines for land disturbing activities in the Auckland Region and TP223 – Forestry operations in the Auckland Region: A guideline for erosion and sediment control l clearly illustrate best practice methods which can be applied to forestry. 

  1. Parliamentary Commissioner for the Environment, 2012, Water quality in New Zealand: Understanding the science, Wellington, available at, at 20

  2. Section 15


  4. Ministry for the Environment, 1997, The state of New Zealand’s environment 1997, Ministry for the Environment, GP Publications, Wellington

  5. Board of Inquiry, 2009, Proposed New Zealand Coastal Policy Statement (2008), Board of Inquiry report and recommendations, volume 1: Findings, recommendations and recommended NZCPS (2009), Wellington, at 9


Last updated at 4:32PM on February 2, 2018