11 February 2026: Health Warning – Harmful Algal Bloom in Te Roto o Wairewa Lake Forsyth
Health New Zealand | Te Whatu Ora has issued a health warning after potentially toxic harmful blue-green algae (planktonic cyanobacteria) surface scum has been observed to be consistently present in Te Roto o Wairewa Lake Forsyth.
People should avoid the lake, and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.
Dr Ramon Pink, Medical Officer of Health for the National Public Health Service, says the algal bloom can produce toxins harmful to humans and animals.
“No one should drink the water from the lake at any time. Boiling the water does not remove the toxin”, he says.
“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”, Dr Pink says.
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”, Dr Pink says.
Environment Canterbury monitors the lake during summer, and the public will be advised of any changes in water quality that are of public health significance.
Facts about cyanobacteria:
- Cyanobacteria occur naturally but can increase rapidly during warmer months.
- Harmful algal blooms are caused by a combination of nutrients in the water (such as nitrogen and phosphorus), and favourable weather conditions (for example as 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.
- Animals that show signs of illness after coming into contact with algal scums should be taken to a vet immediately.
For further details visit:
https://www.lawa.org.nz/explore-data/canterbury-region
https://www.lawa.org.nz/explore-data/canterbury-region/lakes/te-roto-o-wairewa-lake-forsyth
To contact your local National Public Health Service office visit:
https://info.health.nz/locations/canterbury/national-public-health-service-canterbury-west-coast
To keep up to date with current health warnings visit:
https://info.health.nz/health-alerts