The South Taranaki District Council is warning people not to swim in, or eat anything from Lake Rotokare.
There are high levels of blue-green algal bloom in the lake, which can cause skin and eye irritations.
The algae can also produce toxins which can give you a sore throat, asthma, paralysis, vomiting and diarrhoea.
The grounds at Lake Rotokare Reserve are open, but the lake will remain closed until it's cleared by the Taranaki District Health Board.
