People and animals are being warned to avoid Lake Forsyth in Canterbury after potentially toxic blue-green algae was found.

The Community and Public Health division of Canterbury District Health Board issued a health warning on Tuesday after finding a type of cyanobacteria in the lake, south of Christchurch.

The algal bloom can produce toxins harmful to humans and animals, and everyone should avoid contact with the water until further notice, says Canterbury Medical Officer of Health Dr Ramon Pink.

"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips - if you experience any of these symptoms visit your doctor immediately," he said.

No one should drink the water from the lake at any time, and boiling the water does not remove the toxin.

Fish and shellfish can concentrate toxins and their consumption should be avoided. If fish are eaten, remove the gut and liver and wash in clean water.

FACTS ABOUT CYANBACTERIA

* The algae occur naturally but can increase rapidly during warmer months

* If the water is cloudy, discoloured, or has small globules suspended in it, avoid all contact

* Not all cyanobacterial blooms are visible to the naked eye and toxins can persist after the blooms disappear

* Cyanobacterial concentrations can change quickly with environmental conditions, such as the wind