People are being told to stay out of Lake Whangape, near Huntly, because of a harmful bacteria.
The Waikato Regional Council said the cyanobacteria level was almost double what it should be.
The bacteria caused algae that was harmful to humans and animals with reactions including rashes, skin and eye irritations, diarrhoea and vomiting.
Warnings also remained in place for Lakes Waikare, Hakanoa and Lake Rotoroa in Waikato, which also had high levels of the bacteria.
