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 thelake 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.
"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
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.