About 100 eels have been found dead after a chemical compound spilled into a stormwater drain and subsequently into Manukau Harbour.
People are also being warned not to go in the water within 5km from the spill - which covers Māngere Bridge, Taumanu East, Taumanu Central, Taumanu West and Onehunga Lagoon.
In a statement, Auckland Council said 5000 litres of methyl methacrylate was released from storage at a specialist chemical business, Allnex, in Penrose yesterday.
"Staff are assessing what other impact the chemical may have had on marine life and vegetation.
"They are also testing the harbour water. The warning not to swim or go into the water remains and will be extended over the weekend as a precautionary measure."
Methyl methacrylate is used in the manufacture of plastics and adhesives. While it is considered to have low toxicity, it can cause nasal irritation and headaches and has an acrid and fruity odour.
A pollution control team was working on a clean up, the council said, with 20 tons of discharge including water taken away overnight.
Forty nearby businesses have been alerted to the chemical spill, the council said. At this stage, no health issues have been raised with them
Representatives of mana whenua are on site and will be providing input into the clean-up process.
An investigation into how the incident happened is ongoing.
The council said health risk remained low, but if people smelt the chemical they should take a precautionary approach and close doors and windows. The main potential health effects are eye, skin or respiratory irritation.