A health warning is in force for Lake Tutira north of Napier.
Hawke's Bay medical officer of health Nicholas Jones says water tests show the lake has a high concentration of cyanobacteria, or blue green algae.
He says toxins in the algae pose a risk to both humans and animals, and can cause a severe skin rash or an upset stomach.
Mr Jones says people should stay away from the lake and make sure their dogs do too.