A health warning has been issued for Lake Roundabout, near Ashburton, after a potentially toxic algal bloom was found.
Canterbury medical officer of health, Dr Alistair Humphrey, says the planktonic Anabaena cyanobacteria can produce toxins harmful to humans and animals.
He says exposure to the algae may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
Anyone who experience these symptoms should visit their doctor immediately.
Dr Humphrey says boiling the water does not remove the toxin.
The algae are particularly dangerous for dogs.
Eating fish from the lake should also be avoided.
Monitoring of the lake will continue during summer.