An estuary is a body of water where freshwater from rivers and streams flows into and mixes with saltwater from the sea.

A wide range of habitats are found within estuaries; from sub-tidal reefs, inter-tidal mud flats and sea grass beds to landward margin vegetation, including herb fields, saltmarsh, rush-land and sedge-land. These habitats support a diverse range of life; birds, shellfish, fish, and invertebrates, with many species using estuaries for part, or all of their life cycle.

Why are estuaries important?

He taura whiri kotahi mai ano te kopunga tai no I te Pū au.

From the source to the mouth of the river all things are joined together as one.

Environmental values

Estuaries provide critical habitat for species that are valued commercially, recreationally, and culturally. Birds, fish, insects, and other wildlife depend on estuaries to live, feed, nest, and reproduce. Some organisms make estuaries their permanent home while others use them to complete only part of their life cycle. Estuaries provide stopovers for migratory bird species, and many fish in New Zealand pass through estuaries on their way up rivers. Estuaries filter contaminants from the land and so protect the nearby coastal environment and perform an important function for cycling nutrients, much like kidneys of the land.

Economic & recreational values

Estuaries also help keep water clean and protect property from flood and storm damage. The plants and animals in estuaries take up excess nutrients from water and soil and use it for growth, effectively immobilizing pollutants. Tidal marshes, with their dense vegetation and narrow, winding channels, effectively trap sediment and remove it from floodwaters. Fringing marshes, riverine swamps, and other estuarine wetlands, like their upland counterparts, also slow floodwaters and stabilize the shore to prevent erosion. These water-quality and damage control services would cost taxpayers millions of dollars using modern technology, yet estuaries perform them for free.

Estuaries are also important to the commercial and recreational fishing. They either provide essential nursery areas for many commercially and recreationally important fish and shellfish species or the species is reliant on clean estuary filtered water entering the near shore coast.

It is difficult to measure the dollar value of the many functions provided by estuaries but some scholars have estimated the economic value of ecosystem services to be very high.

Estuaries are important recreational areas. People visit estuaries year-round to boat, swim, watch birds and other wildlife, and fish.

Cultural values

Estuaries or Ngutuawa are a highly valued part of a catchment. For Māori, historically and today, estuaries provide a wide selection and abundance of mahinga kai (foods) which includes fish, shell fish and birds. Estuaries are breeding areas. They are pathways to the sea and inland for both people and fishes. These are reasons why Māori built kāinga (villages) beside them.

The Ngāi Tahu catchment management philosophy, ki uta ki tai (mountains to the sea) informs us all parts of the land and water within a catchment are connected.

Poor estuary health and poor harvest can indicate the poor health of the river system.

Kaitiakitanga, described as stewardship is a responsibility of people to look after the treasures of the land, air, waters and sea for us and future generations.

Treasure our estuaries.

The issues

As the population increases, our coastal areas come under pressure and estuaries often suffer from human activities such as run-off from agriculture and wastewater discharges. Sedimentation, excessive nutrients, toxic contaminants, disease risk and habitat loss are all major issues currently facing some estuaries in Southland and New Zealand.

Currently specific official assessments for estuary state are not available for New Zealand. However it is clear from State of the Environment monitoring that some estuaries are in a poor state whilst others are in very good condition. It all depends on the combination of sensitivity of the particular estuary and pressure from land use.

What we are doing

Environment Southland has been monitoring six estuaries, two harbours and one lagoon as part of its estuary programme.

In 2000, monitoring started on the four largest estuaries on the south coast; Waikawa Harbour, New River, Jacobs River and Toetoes Estuaries. Haldane Estuary and Bluff Harbour were added by 2005, and in 2008 monitoring began in Waimatuku Estuary, Waiau Lagoon and Freshwater Estuary on Stewart Island. Freshwater Estuary is monitored as a 'control' or unmodified estuary, to compare other estuaries to.

Environment Southland follows the Estuary Monitoring Protocol which has four parts; broad scale habitat mapping, fine scale sediment analysis, algal bloom mapping and vulnerability/risk assessments. We measure the things below to give an indication of the overall health of the system.

What we measureIndicators​
  • Muddiness
  • Area of soft mud
  • Sedimentation build-up​
Habitat quality​
  • Extent of seagrass beds
  • Estuary invertebrates​
Sediment contamination​
  • Heavy metal toxicity​
Nutrient enrichment​
  • Macroalgae cover
  • Sediment oxygen levels
  • Sediment nutrients concentrations​


The latest reports on our estuary monitoring programme are available for download below.

Page reviewed: 03 Feb 2017 11:20am