Harbour and estuary ecology state and trends in Tāmaki Makaurau / Auckland to 2023. State of the environment reporting. 

Executive summary

Tāmaki Makaurau / Auckland is predominantly marine, with varied seascapes making up roughly 70 per cent (11,117 km²) of the region’s territorial area. Estuaries (ranging from large, traversable harbours to small estuaries and inlets) make up two thirds of the coastline and are the focus of this report.

Population growth and the intensification of land use puts pressure on the natural environment. In estuaries, these pressures mostly occur in the form of pollution from excess sediment, nutrients and contaminants (like metals and organic chemicals), and modification of the natural coastline. This report presents technical information describing the current ecological state (or ‘health’) of the estuaries in Tāmaki Makaurau and how this has changed over time, making connections to the influence of land-derived pressures.

Monitoring focused on benthic ecosystems (those associated with the seafloor rather than the water column) in the intertidal zone (the area that is periodically covered and uncovered by the tides) because of their high ecological value and because their physical and biological components are responsive to land-derived pressures. We sampled sandflat characteristics (such as the proportion of mud and organic content) and sediment-dwelling invertebrates (i.e. macrofauna), which have limited mobility and known preferences and sensitivities that make them useful as indicator species.

A total of 172 monitoring sites from across 15 estuaries had benthic ecology data available for assessment of current state (drawing from Auckland Council’s Harbour Ecology, East Coast Estuaries Ecology and Regional Sediment Contaminant Monitoring Programmes). According to the Combined Health Score, only 2 per cent of the monitored sites were in ‘Excellent’ overall health and 22 per cent were ‘Good’. Sites most commonly had ‘Fair’ (34 per cent) or ‘Marginal’ (30 per cent) health and 12 per cent of sites were ‘Poor’.

Excess land-derived sediment (silts and clays which are referred to as ‘mud’ in the estuary) continued to be the most prevalent and severe pressure on benthic environments across Tāmaki Makaurau. All sites that had ‘Excellent’ overall health also had less than 3 per cent sediment mud content, and all sites that were in ‘Poor’ health had at least 30 per cent mud content. Every monitored estuary exhibited at least moderate impacts from sediment and long-term degrading conditions at one monitoring site or more. Fewer estuaries (eight out of 14) were impacted by metal contamination and in several locations (namely Māngere Inlet, Waitematā Harbour tidal creeks and Tāmaki Estuary) the impact of metals on the health of the benthic community had decreased over the long-term. Ecological health may have been impacted by excess nutrient concentrations at sites in seven of the monitored estuaries (up from three in the last state and trends report), though this requires further investigation given limitations of the (indirect) indicators used to identify nutrient impacts.

Health state and trends varied by location. It was most common for ecological condition to have degraded and for overall health to be poor (as was the case for Mahurangi Harbour, Pūhoi River and Mangemangeroa Creek, for example). Management interventions need to focus on reducing the pressures on these estuaries, such as minimising the input of pollutants from land-based activities. The main basin of Manukau Harbour, Whangateau Harbour and sites beyond the estuary mouth in Tūranga Creek had limited degrading trends and were in largely good ecological health. For these locations, management efforts should focus on protection and conservation. A similar approach might be recommended for estuaries where ecological health was mostly poor but improving trends were detected (like Tāmaki Estuary and the tidal creeks of Manukau Harbour), though active restoration may increase their recovery rate.

Auckland Council technical report, TR2025/15

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