Hawke's Bay Regional Council is advising the public to avoid contact with Lake Tūtira.
The Council sampled Lake Tūtira today, as part of regular monthly monitoring.
Freshwater scientist Dr Andy Hicks believes the current algal bloom is particularly nasty and conditions are not expected to improve anytime soon.
"The lake should start to 'turn over' or mix again in June as surface waters cool off and become the same density as the bottom waters," says Dr Hicks.
"This will make conditions less favourable for blue-green algae and so the bloom will hopefully subside. The current bloom is potentially toxic, so contact with the water should be avoided," he adds.