People in the Auckland suburbs of Onehunga and Penrose are being warned to keep their doors and windows closed after a chemical spill leached into the Manukau Harbour.
On Thursday 5000 litres of methyl methacrylate spilled at a business on Industrial Rd - it entered a storm drain and went straight into the harbour.
Used in the manufacture of plastic and adhesives, methyl methacrylate is considered to have low toxicity - but it can cause headaches and nasal irritation.
Residents in the area have been told to close their windows and doors if they can smell the fruity odour of the chemical.
Auckland Council Manager of Environmental Health Mervyn Chetty told Newshub teams will be out on Friday morning assessing the risk.
"The main health impacts are eye, skin or respiratory irritation," he said.
"There could be some environmental impacts."