Groundwater quality state and trends in Tāmaki Makaurau / Auckland 2017-2024. State of the environment reporting

Executive summary

Auckland Council undertakes long-term monitoring of groundwater quality as part of its State of the Environment reporting. The monitoring is used to assess state and detect trends in physical, chemical and microbiological properties of groundwater in some of the region’s aquifers. The results enable council to track changes in groundwater over time. They can be used to identify potential contamination and risk, and evaluate the effectiveness of council initiatives, policies and groundwater management strategies. 
Several pressures affect groundwater quality in the Auckland region including agricultural and horticultural production, urban stormwater discharge and water abstraction for drinking and irrigation purposes.

The groundwater quality programme started in 1998 with the purpose of monitoring aquifers in specific parts of the Auckland region where groundwater use is very important, or where quality is considered at risk (now or in the future). Site locations and monitoring frequency have changed over time, but since 2014, quarterly sampling at 18 sites has occurred, with three sites added since 2020.

This report provides an overview of the state and identifies trends in groundwater quality at these sites over the most recent seven-year period (2017-2024). This short trend period begins after a change in laboratory, which was associated with step-changes in the results for some parameters.
High nitrate concentrations in the shallow basalt aquifers and springs of the Pukekohe area remain a key concern. Median nitrate-N concentrations at some sites exceed the drinking water Maximum Acceptable Value (MAV). These levels reflect long-term impacts from intensive horticultural activity and are unlikely to decline quickly due to long groundwater residence times (18-36 years). These aquifers also contribute nitrate-laden baseflow to local streams, some of which exceed national toxicity thresholds. While nitrate levels appear stable, no improving trends have been observed.

Urban groundwater in Auckland’s isthmus shows signs of contamination from stormwater infiltration via soak pits. E. coli and nitrate levels are consistently elevated, as is lead at one site, likely from urban sources.

In areas underlain by sedimentary geology, elevated soluble iron concentrations were common, with increasing trends at several sites, potentially linked to declining dissolved oxygen levels. Overall, the influence of underlying geology and redox conditions have been suggested as influencing variability in metal and ion concentrations, but this remains speculative in the absence of site-specific geological investigations.

Auckland Council technical report, TR2025/21

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