Swimmers are being urged to keep out of Lake Tutira after an algal bloom sampled last week has been found to contain potentially toxic cyanobacteria.
Cyanobacteria is the toxin that led to the death of 4-year-old labrador Marley after she drank from the Tukituki River.
The Hawke's Bay Regional Council environmental science staff sent water samples from the lake to the Cawthron Institute.
Cawthron had assessed that the water was dominated by a potentially toxic species of cyanobacteria algae, Dolichospermum, senior environmental scientist Anna Madarasz-Smith said.
It was at such a level that required an alert to the public to stay out of the water.
Although a bloom before Christmas was assessed as non-toxic, as summer had progressed the dominant species had changed to one that could produce toxins, she said.
"People are advised to avoid any contact with the water at Lake Tutira."
Hawke's Bay District Health Board Medical Officer of Health Nicholas Jones said people should stay out of the water as toxic blooms could develop quickly and skin contact could lead to skin rashes or eye irritation.
"More serious effects can occur if toxins are consumed. There may also be some risk with eating any fish from the lake, and if people want to eat fish, it should be thoroughly gutted and cleaned," Mr Jones said.
There is permanent signage at Tutira warning visitors about the health risks of algal blooms and the regional council is continuing a project of computer modelling the lake catchment to assess possible solutions to improve lake water quality.
The council confirmed there had been no more incidents of pets being poisoned by the water since Marley's death.