The Canterbury District Health Board is warning people to keep out of Lake Forsyth on Banks Peninsula near Christchurch.
The DHB says there is a bloom of toxic blue-green algae in the lake which means it is unsafe to swim in.
The algae looks like green globules and is especially toxic to dogs.
The DHB warns that boiling the water will not remove the toxin and it also affects shellfish.
The bloom is the second at the lake since December.