A health warning has been issued for Lake Rotokare, east of Eltham in Taranaki.
The Medical Officer of Health for Taranaki, Dr Greg Simmons, says tests have shown the lake contains high levels of cyanobacteria.
He says people shouldn't use the lake for any purpose, or allow pets to enter or drink the water.
Warning notices are being put up by the South Taranaki District Council.
Cyanobacteria is an alga capable of producing toxins, most commonly causing skin and eye irritation. It can also be responsible for asthma-like symptoms, liver damage, and diarrhoea.