Marine sediment contaminant state and trends in Tāmaki Makaurau / Auckland 2004-2023. State of the environment reporting. 

Executive summary:

Contaminants can accumulate in the sediments of our harbours, estuaries, and beaches. When present at elevated levels, metals (such as copper and zinc), nutrients (like nitrogen and phosphorus) and organic compounds (like pesticides and hydrocarbons) can cause both acute and long-lasting harm to coastal ecosystems. These pollutants can reduce the diversity and abundance of animals that live on and within the sediment, disrupting key ecosystem functions and degrading ecological communities. Contaminants originate from various human activities and can enter the marine environment through multiple pathways, such as streams, urban stormwater, and industrial discharges. Understanding the distribution and levels of contaminants in marine sediments provides a useful marker of land use impacts on aquatic environments and ecosystem health.

Auckland Council monitors marine sediment contamination through the Regional Sediment Contaminant Monitoring Programme (RSCMP). The RSCMP assesses near-shore contamination and tracks how concentrations change over time. The programme focuses on key urban-related metal pollutants: copper, lead, zinc, arsenic, and mercury. Monitoring is carried out as part of fulfilling Auckland Council’s legislative obligations to monitor and report on the state of the environment, and information gained is used to identify issues and inform policy development and environmental decision-making.

This report covers the period 2004 to 2023 and provides an assessment of contamination at 97 sites across the region. It compares concentrations to sediment quality guidelines to assess their potential impact on marine sediment ecosystems, and examines temporal trends in levels of copper, lead, zinc, and mud content.

Contaminant concentrations across Tāmaki Makaurau vary significantly. Sites are categorised using a traffic light system: ‘red’, ‘amber’ or ‘green’. Most sites show relatively low contaminant levels, with three-quarters rated as 'green’. By this measure, the impact on animals living on and within the sediment is expected to be minimal. The remaining sites are classified as either 'amber', where levels are moderately elevated and ecological impacts may begin to appear, or 'red', where degraded ecology would be expected as a result of elevated concentrations.

Both current and historical land use in surrounding catchments, along with an estuary's physical characteristics (such as hydrodynamics, sediment accumulation, and sediment texture), influence contaminant levels at monitoring sites. While contaminant concentrations are generally fairly low across the region, elevated levels are found in areas with intensive urban and industrial use, particularly where such activities have been ongoing for a long time.

Contamination patterns are consistent with previous reporting. The highest levels are found along the southern coastline and sub-estuaries of the Central Waitematā, the Tāmaki Estuary, and, to a lesser extent, the Upper Waitematā and Māngere Inlet. Outside of the urban areas of Auckland and in the more exposed bodies of harbours, concentrations are low.

Zinc remains a contaminant of concern and is the metal most frequently detected at elevated concentrations. Copper and mercury are generally low, though levels can be moderately elevated in some areas. Lead is mostly at concentrations below those associated with ecological impacts, and arsenic does not appear to be causing significant effects at any sites. The contaminants measured are rarely elevated on their own, and there is a strong correlation between copper, lead, zinc, and mercury levels, suggesting a common major pathway (likely urban stormwater) into Auckland's estuaries. Mud content influences contaminant concentrations, with contaminated sites typically containing muddy sediments that are known to bind toxins, allowing them to accumulate.

At most sites, contaminant concentrations showed little change during the reporting period. Where changes were observed, these were typically slow and gradual, rather than abrupt. Overall, copper and lead concentrations show modest declines, suggesting some improvement in contamination levels, while zinc shows a slight increasing trend. General improvements in lead levels have been observed in successive reports since 2012, reflecting both continued progress and the gradual recovery of contaminated sediment. Trends appear to largely be site and metal specific. In the Māngere Inlet in the Manukau Harbour several sites are showing decreasing concentrations of copper and lead, suggesting broader improvements across this area.

The relative stability of metal concentrations in Auckland is encouraging, especially as urban pressures have risen dramatically over the monitoring period. While this may suggest that to date, these growing pressures are being offset by improvements in areas like vehicle emissions and stormwater management, maintaining broad spatial monitoring remains important. It allows for the detection of potential shifts in contaminant distribution, driven by expanding and intensifying urbanisation, changes in climate, and evolving sediment dynamics, and supports the ongoing evaluation of efforts to reduce contaminant inputs.

Auckland Council technical report, TR2025/12

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