Health New Zealand Te Whatu Ora

Health New Zealand Te Whatu Ora has issued a health warning after potentially toxic harmful blue-green algae (planktonic cyanobacteria) were found in Lake Hood.

People should avoid the lake and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.

The type of cyanobacteria that is currently present in high concentrations is:  Dolichospermum (formerly called Anabaena) which can be present as cloudy, discoloured water and/or green globules floating in the water column or form surface scums or sheens.

Dr Cheryl Brunton, Medical Officer of Health for the National Public Health Service, says the algal bloom can produce toxins harmful to humans and animals.

“People should avoid contact with the water until further notice. No one should drink water from the lake at any time. Boiling the water does not remove the toxin.

“Fish can concentrate toxins and their consumption should be avoided. If fish are eaten, remove the gut and liver and wash in clean water.

“When a bloom is present in a lake, there is a possibility of cyanobacteria and toxins being transported downstream. People are advised to avoid contact with the downstream water bodies.

“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."

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.

The lake itself is not closed, but Ashburton District Council asks people to follow the advice of the health warning.

In April 2025, the lake was closed when toxic cyanobacteria known to be harmful, Microcystis was detected, and there were two substantiated cases of people falling ill from lake exposure.

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:
lawa.org.nz/explore-data/canterbury-region

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