Health New Zealand has issued a health warning after potentially toxic blue-green algae (planktonic cyanobacteria) was detected in Lake Pegasus.

Residents and visitors are advised to avoid the lake, and animals, particularly dogs, should not be allowed near the water until the warning is lifted.

The algae identified is the Tolypothrix species, which can produce harmful toxins.

Dr Cheryl Brunton, Medical Officer of Health for the National Public Health Service, said the algal bloom poses serious health risks.

“People should avoid contact with the water until further notice,” Dr Brunton said.

“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 inform them of any contact with lake water.”

Drinking lake water is strictly prohibited, as boiling does not remove the toxins.

Animals that show signs of illness after contact with the water or algal mats should be taken to a vet immediately.

Environment Canterbury will monitor the lake weekly throughout the summer. Any changes in water quality that impact public health will be communicated to the public.

•Cyanobacteria occur naturally but can increase rapidly during warmer months.

•Algal blooms are caused by a combination of nutrients in the water (e.g., nitrogen and phosphorus) and favourable weather conditions, such as increased temperature and calm days.

•If the water appears cloudy, discoloured, or contains small globules, avoid all contact.

•Not all blooms are visible, and toxins can remain even after the algae are gone.

•Concentrations can change quickly with environmental shifts, such as wind.

For more information, visit LAWA Canterbury.