River water quality current state and trends in Tāmaki Makaurau / Auckland to 2024. State of the environment reporting. 

Executive summary

This report presents the current state of river water quality in Tāmaki Makaurau, assesses it against the National Policy Statement for Freshwater Management (NPS-FM) 2020 National Objectives Framework (NOF) water quality attributes and additional draft regional water quality attributes, and explores how water quality has changed over the past seven years. It supports the State of the environment 2025 report for the Auckland region, Te oranga o te taiao o Tāmaki Makaurau – The health of Tāmaki Makaurau Auckland’s Natural Environment in 2025: a synthesis of Auckland Council State of the Environment reporting.

Water quality can be affected by land use activities, point and diffuse source discharges, land and stream channel erosion, as well as seasonal and climatic variability. River water quality is monitored monthly at 37 sites across Auckland, covering a range of parameters including water temperature, dissolved oxygen, nutrient concentrations (nitrogen and phosphorus), metal contaminants (copper and zinc), suspended sediment and water clarity, and faecal indicator bacteria (Escherichia coli). 

The current state was assessed based on the five-year period from 1 July 2019 to 30 June 2024. Trend analysis followed methodology established for national water quality reporting which assesses the confidence in the trend direction and provides an estimate of the magnitude of change. Assessment focused on the seven-year period from 1 July 2017 to 30 June 2024 as multiple methodology changes in 2017 presented a limitation on long-term time series analysis.

Several historically significant climatic events occurred over these state and trend periods including a severe drought (2020) and several extreme rainfall and flooding events (2023). Water quality monitoring was representative of the flow conditions that occurred, although extreme flow events were not fully captured because of health and safety considerations.

Water quality was generally in the best condition in streams draining native forest catchments. Water quality was progressively worse in streams flowing through predominantly rural, and urban catchments, with urban streams in the poorest condition. Variation in some aspects of water quality including water clarity and total suspended solids was more clearly explained by climate and underlying geology than land cover.

All monitored rural and urban waterways were impacted by high E. coli levels that indicate faecal pollution. Most rural and urban waterways have elevated nutrient concentrations that can influence instream ecosystem health and function.

Multiple streams within the wider Pukekohe area failed the national bottom line for nitrate toxicity. This is a long-standing issue associated with nitrate contamination in several of the underlying shallow aquifers.

Multiple streams within urban areas failed the proposed regional bottom line for chronic zinc toxicity (i.e. consistently occurring exposure) and occasionally exceeded acute toxicity guidelines.

There were key changes to the assessment of chronic copper and zinc toxicity regional attributes. This change in assessment meant that the risk from chronic, long-term copper exposure on aquatic life was less than we had previously reported. New Zealand guideline values for acute (short-term) toxicity effects from copper and zinc on aquatic life have recently been developed and monthly water quality observations were also compared to these guidelines.

Several monitored streams failed the national bottom line for visual clarity (as a proxy for suspended sediment) including some rural sites and one urban site. This is likely due to a combination of catchment-specific factors, including environmental conditions such as climate and underlying geology.

Overall, there was wide variation in trend direction, and magnitude across river sites, land cover types, and among different measures of water quality. This highlights the complexity of interactions between land use activities, different contaminant pathways, and climatic influences.

Generally, there was a higher proportion of degrading trends where water quality was most impacted (i.e. bad and getting worse). In one catchment with increasing urban development (Vaughan Stream at Long Bay) there are indications that most measures of water quality are at least being maintained.

There were many site-specific issues, and multiple sites where a higher magnitude of change (improving or degrading) was observed across multiple measures of water quality. Potential further follow-on investigations have been identified for several of these locations.

The findings of this report can be used to inform current state and trends as required for NPS-FM implementation with a focus on ecosystem health water quality and human contact attributes. This also complements and informs understanding on other compulsory values under this policy. This report contributes to the regional evidence base to inform the effectiveness of policy initiatives.  Long term, regional information provides important context for operational (and restoration) activities and other monitoring and research in the region by providing the expected range of various measures of water quality across different broadscale pressures, including seasonal variability.  

Auckland Council technical report, TR2025/20

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