Aucklanders are being told not to go into the water within 5km of a stormwater drain which carried 5000 litres of a spilled chemical from Penrose to Manukau Harbour.
The advice covers shorelines at Māngere Bridge, Taumanu East, Taumanu Central, Taumanu West and Onehunga.
Auckland Council says methyl methacrylate was released from a specialist chemical business on Industrial Road in Penrose.
The chemical, normally used to make plastics and adhesives, ran into a stormwater drain and out into the Manukau Harbour.
Health officials are warning workers in the Penrose and Onehunga industrial area they may need to keep windows and doors shut this morning in the wake of yesterday's spill.
Similar advice was given to local householders last night.
The chemical has acrid and fruity odour, and can cause eye, skin or respiratory irritation.
Auckland Council says it is investigating the circumstances around the incident and anyone with any further concerns should contact Healthline on 0800 611 116 for advice.