Health New Zealand has issued a health warning after potentially toxic algae was discovered in Lake Hood, near Ashburton.

People are being urged to avoid the lake, and animals—particularly dogs—should be kept well away from the water until the warning is lifted.

The high concentrations of planktonic cyanobacteria detected in the lake is Dolichospermum (formerly known as Anabaena). It can appear as green globules floating in the water, or form surface scums and sheens.

Public Health Medicine Specialist Dr Annabel Begg said the algal bloom can produce toxins harmful to both humans and animals.

“People should avoid contact with the water until further notice,” Dr Begg 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 let them know if you’ve had contact with the lake water.”

Drinking water from the lake is also strongly discouraged, as boiling the water does not remove the toxins.

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

Dr Begg said fish and shellfish from the lake could concentrate toxins and should not be eaten. “If fish are eaten, remove the gut and liver and wash in clean water.”

She warned there is a risk of toxins being transported downstream and advised people to avoid contact with any downstream water bodies.

Environment Canterbury monitors Lake Hood weekly over summer and said the public would be updated if the water quality changes.

Cyanobacteria occurs naturally but can quickly increase during warmer months when nutrient levels are high, and weather conditions are calm. Health officials said not all blooms are visible, and toxins can remain in the water even after the blooms disappear.

Further information is available at: www.lawa.org.nz/explore-data/canterbury-region/