The health warning over Lake Tutira has been extended as higher levels of cyanobacteria have been detected in its water by the Hawke's Bay District Health Board.
People should not swim in the lake and avoid contact with the lake water, the DHB said. Some cyanobacteria species produce toxins (cyanotoxins) which pose a risk to humans and animals when consumed or by direct contact during recreational activities.
Medical Officer of Health Nicholas Jones said the Hawke's Bay Regional Council had found cyanobacteria in the lake in April and a warning was issued.
"This warning had not been lifted because testing continued to show the bloom comes and goes and recent testing confirmed high levels of the toxin."
Dr Jones said the DHB wanted to reinforce the warning because the bloom was becoming worse and more people may be tempted to use the lake as warmer weather sets over the region.
He also warned if people eat fish or shellfish from the lake they should avoid eating the gut and organs altogether as these parts of the fish accumulated toxins.
The DHB and regional council staff had agreed the lake needed to be classified as having "an ongoing risk".
"Testing can help us identify risk at a particular time but because cyanobacteria blooms come and go quickly at the lake DHB staff can't use the results to provide accurate advice to the public about cyanobacteria risk in between samples.
"Toxins produced by cyanobacteria could also be in the water even after the visible bloom has disappeared."
New permanent signage at the lake would highlight the long-term risk of cyanobacteria as well as the risks from bacteria contamination and duck itch.
Cyanobacteria are single cell creatures that live in the water and have characteristics in common with bacteria and algae.
In warm, nutrient rich conditions free-floating cyanobacteria cells can multiply quickly to form what are known as algal blooms.
Large numbers of cyanobacteria can also grow as mats on river and lake beds and these mats can sometimes detach and float to the surface.