A warning has been issued for two popular swimming lakes near Dunedin because of algal blooms.
Otago Regional Council was told on Wednesday about mats of blue-green algae in the Tomahawk lagoon in Dunedin and at Lake Waihola, south of the city.
Tests show the algae is a plant called Anabaena lemmermannii which can produce toxins that make people ill and cause animals to die.
Environmental information director John Threlfall said people should not swim in the lakes or allow dogs near the water.
Algal blooms have occurred at the lakes before, but it is unusual to find them so early in the summer, he said.