Harmful algal bloom in Lake Hood
Canterbury
24 March 2026
Health New Zealand | Te Whatu Ora has lifted its harmful algal bloom health warning issued for Lake Hood.
Recent water testing at Lake Hood has shown the quantity of potentially toxic harmful algae (planktonic cyanobacteria) in the lake has reduced and concentrations are now below levels that are of concern to public health.
Dr Claire Salter, Medical Officer of Health for the National Public Health Service, says “the public will be informed if testing shows that concentrations have increased and there is a risk to public health again”.
For further details
Canterbury region — Land Air Water Aotearoaexternal link
14 January 2026
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," she says.
“Fish can concentrate toxins and their consumption should be avoided. If fish are eaten, remove the gut and liver and wash in clean water," Dr Brunton says.
“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," she 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 Brunton 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.
Further information
Canterbury region — Land Air Water Aotearoaexternal link