After sightings of water discolouration last Friday, samples taken from Lake Tarawera have confirmed the presence of blue-green algae.
Samples taken from the middle of Lake Tarawera and at Te Rata Bay near Hot Water Beach have identified blue-green algae in the lake, said Bay of Plenty Regional Council science manger Rob Donald.
He said the lake was at "amber alert" level which means the situation will continue to be monitored but no formal health warning was justified at this stage.
"The blue-green algae identified are potentially toxic but the levels of algae are below health guidelines. We recommend that people do not swim in the water if it is discoloured."
The blue-green algae have arrived recently in New Zealand and are now present in lakes throughout the North Island. Algal blooms will cause the water to appear green and cloudy and at times the algae colonies will be visible as small green specks.