Executive summary

Contaminants such as copper, lead, zinc, arsenic, and mercury can accumulate in the sediments of our harbours, estuaries, and beaches. They originate from a range of different activities and land uses including vehicle tyre and brake wear, industrial discharges, use of agrichemicals, and the breakdown of some building materials. When it rains, these pollutants can wash into stormwater networks and waterways, ending up in the marine environment. The build-up of these contaminants can affect ecological health by reducing the abundance and/or diversity of animals living in the sediment. This results in degraded communities that are dominated by the remaining few species that are tolerant of higher contaminant levels, with flow-on effects for the natural functioning of these ecosystems. Understanding the distribution and level of chemical contaminants in marine sediments provides a useful marker of land use impacts on aquatic receiving environments and ecosystem health.

Auckland Council’s Regional Sediment Contaminant Monitoring Programme (RSCMP) monitors chemical contaminant levels in marine sediments at approximately 80 sites across the region’s harbours and estuaries. Monitoring follows a temporally nested design, with sites sampled on a rotational basis every three or six years. This report summarises the state of sediment metal contamination and changes over time in state at 14 Manukau Harbour sites sampled in 2024.

Total recoverable metals in the <500μm sediment fraction were analysed for copper, lead, zinc, arsenic (a metalloid), and mercury. One composite sample from each site was also analysed for particle size distribution to characterise sediment grain-size composition. Analysis of quality assurance results indicated that the metals and particle size distribution data collected in 2024 were largely within acceptance criteria and are considered suitable for use in the RSCMP.

Contaminant state is assessed against sediment quality guidelines that indicate potential impacts on marine sediment ecosystems. Sites are classified using a traffic light system: red (elevated concentration with likely ecological effects), amber (moderate concentration with possible effects), and green (low concentration with minimal or no effects).

Results from sampling undertaken in 2024 showed that overall, metal contamination levels in the Manukau Harbour were low. Only one site, Anns Creek, exceeded sediment quality guidelines for zinc, receiving an ‘amber’ grade; all other sites fell within the green category, indicating minimal impact on animals living on and within the sediment. Comparison with previous monitoring shows that contaminant state has remained stable across most sites, with notable improvements in the Māngere Inlet, where copper, lead, and zinc levels have improved from red or amber to predominantly green.

Auckland Council technical report, TR2025/33

December 2025