The Canterbury District Health Board is warning people to stay away from Lake Pegasus.
Canterbury DHB Medical Officer of Health Dr Cherly Brunton issued a public health warning for the lake on Monday after potentially toxic algae, also known as planktonic cyanobacteria, was found there.
The public should avoid the lake and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted, she said.
The algal bloom can produce toxins harmful to humans and animals.
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"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 and please let your doctor know if you have had contact with the lake water."
No one should drink the water from the lake at any time. Boiling the water does not remove the toxin, Dr Brunton said.
Animals that show signs of illness after coming into contact with algal mats or scums should be taken to a vet immediately.
Environment Canterbury monitors the lake weekly during summer and the public will be advised of any changes in water quality that are of public health significance.
Stuff has reported it is the sixth summer in a row a health warning has been issued for the lake.
A warning has been issued for the man-made lake every summer since 2014/15.