Lake Hood has been closed to the public until further notice due to an algal bloom and the risk to health posed by cyanobacteria toxins.

the Ashburton District Council made the decision on Thursday afternoon.

A health warning to avoid contact with the lake water was issued on March 20because of high levels of potentially toxic blue-green algae, and more recent testing has shown the presence of a new species of algae which is known to be toxic.

Chief Executive Hamish Riach said the council was closing Lake Hood to the public because the cyanobacteria posed a genuine health risk.

“There have been two substantiated cases reported by GPs of patients with typical symptoms of exposure to cyanobacteria and council wishes to protect the public from the clear public health risk.

"The continued use of the lake in its current state poses an unacceptable health risk.

“Water testing at different points around the lake and canals shows varying levels of the algae but we are treating the water body as one because it is all interconnected.”

The jumping platform, off the lake extension, is also closed.

The council has informed the organisers of a national waterski event planned at Lake Hood on April 15-21 of the closure, and they are investigating an alternative venue in the Ashburton district.

Riach said the closure was supported by Environment Canterbury and Health New Zealand, who had issued the health warning last month.

“Lake Hood will continue to be tested regularly and we’ll keep the community up to date with any changes.

"Council, as managers of the lake, will keep working on ways to improve the water quality.”

Signs alerting residents and visitors about the closure will be installed around the lake today and the gate leading to the main boat ramp will be closed.

Health New Zealand said people should avoid contact with the lake water, and animals (especially dogs) should not be allowed near the water until the health warning has been lifted.

Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips. People who experience these symptoms should visit their doctor and let them know if there has been contact with lake water.

Animals that show signs of illness after coming into contact with lake water should be taken to a vet.