People and animals should avoid drinking, touching or swimming in water from Canterbury's Lake Ellesmere (Te Waihora), health authorities say.
The Community and Public Health division of Canterbury District Health Board has issued a health warning after high concentrations of blue-green algae (planktonic cyanobacteria) were found in the lake.
People and animals, particularly dogs, should avoid contact with the water until the warning is lifted.
The algae occur naturally but can increase rapidly during warmer months. Water that is cloudy, discoloured or contains small globules should be avoided.
Not all cyanobacterial blooms are visible to the naked eye and toxins can persist after the blooms disappear.
Cyanobacterial concentrations can change quickly with changing environmental conditions such as wind.
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CDHB spokesman Dr Ramon Pink said boiling the water does nothing to remove the toxin.
"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."
Animals that show signs of illness after coming into contact with algal mats should be taken to a vet immediately.
Fish and shellfish can concentrate toxins and should not be eaten.
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.