Coastal and estuarine water quality state and trends in Tāmaki Makaurau / Auckland 2024. State of the environment reporting

Executive summary

This report is one of a series supporting the Auckland region’s State of the Environment reporting. It provides regional and local coastal water quality updates to inform sustainable management, policy evaluation, and progress towards the Auckland Plan 2050 goals.

Coastal and estuarine water quality is monitored monthly at 31 sites across three main harbours, two large estuaries, and several Hauraki Gulf shoreline locations. We measure physical and chemical parameters, focusing on nutrients and water clarity, which can be influenced by land use, discharges, and seasonal or climatic variation.

We report on water quality state for a five-year period from July 2019 to June 2024. The trend reporting period for most parameters was reduced to a seven-year period from July 2017 to June 2024 due to the presence of step changes in the data associated with a change of laboratory in June-July 2017. Longer trend periods of 10 and 15-years up to June 2024 were included for parameters measured in the field. Comparison of the different trend periods indicated impacts of heavy rain events observed in the Auckland region in 2023 on the shorter trend period. Consequently, trend results for the seven-year period should be interpreted as indicative only.

Water quality was better at sites at the open-coast and at outer estuarine sites with high exposure to marine waters. At these sites water quality was generally rated as “good” or “excellent”. Impact on water quality in the form of nutrient pressures and reduced water clarity became stronger at more sheltered estuarine sites with higher freshwater impact. At these sites water quality was largely classified as “fair”. Water quality was poorest at sites closest to freshwater sources and sheltered areas in the upper estuaries. Water quality at these sites was most often classified as “marginal” or “poor”. Overall water quality classifications either improved from or remained the same as previous reporting periods.

Trend results revealed signs of improvement in water quality at many sites across the region. More than 75 per cent of sites showed improving trends for total nitrogen, dissolved reactive phosphorus and turbidity. Ammoniacal nitrogen and total suspended solid trends were improving at more than 70 per cent of sites. Improving trends for chlorophyll a were found at more than half of all sites.

Individual water quality assessments for six areas in the region showed localised impacts on water quality with signs of nutrient pressures and/or low water clarity in the Manukau Harbour, the upper Waitematā Harbour and at the Kaipara River Mouth. Degrading trends for nutrient parameters were found for some sites in the Manukau Harbour and the upper Waitematā Harbour. At the Kaipara River Mouth, there were improvements in water quality, especially for water clarity parameters.

To better understand the extent of ecological impacts we recommend targeted investigations to characterise daily and seasonal fluctuations of algal growth and dissolved oxygen for areas showing signs of nutrient pressures.

Our report shows signs of influence on coastal water quality by climatic events, catchment activities, freshwater inputs, and marine processes. To identify specific drivers and support effective management, monitoring nutrient and sediment loads at terminal river reaches and adding oceanic water quality sites should be considered.

Auckland Council technical report, TR2025/19

September 2025

See also

Te oranga o te taiao o Tāmaki Makaurau. The health of Tāmaki Makaurau / Auckland’s natural environment in 2025. A synthesis of Auckland Council’s state of the environment reporting