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The lake should be avoided and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.
Canterbury Medical Officer of Health Dr Cheryl Brunton said the algal bloom can produce toxins harmful to humans and animals.
"People should avoid contact with the water until further notice," 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 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."
Animals that show signs of illness after coming into contact with algal mats or scums should be taken to a vet immediately.
Fish and shellfish can concentrate toxins and their consumption should be avoided. If fish are eaten, remove the gut and liver and wash in clean water.
A similar health warning was issued for the lake in January and lifted in May.
Cyanobacteria quick facts:
- Cyanobacteria occur naturally but can increase rapidly during warmer months.
- Algal blooms are caused by a combination of nutrients in the water (such as nitrogen and phosphorus), and favourable weather conditions (e.g. increased temperature, calm days).
- If the water is cloudy, discoloured, or has small globules suspended in it, avoid all contact.
- 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 (e.g. wind). If a health warning is in place avoid contact with the water.