A health warning has been issued after potentially toxic algae were found in a North Canterbury river.

People and animals, particularly dogs, should avoid the Cust River near Skewbridge Rd until the health warning, imposed by the Community and Public Health division of the Canterbury District Health Board, has been lifted.

Canterbury Medical Officer of Health, Dr Alistair Humphrey, says the algae (benthic cyanobacteria) looks like dark brown to black mats and can produce toxins harmful to people and animals.

"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips," he said.


"If you experience any of these symptoms, visit your doctor immediately, also let your doctor know if you've had contact with dark brown/black algal mats or water in this area."

Humphrey says no-one should drink the water from the river at any time.

"Even after boiling the water from the river, it does not remove the toxin therefore should not be consumed," Humphrey says.

Pets should also be taken to a vet immediately if they are showing signs of illness after coming into contact with algal mats.

"People and animals should remain out of the waterways until the warnings have been lifted," Humphrey says.

Environment Canterbury is monitoring the sites and the public will be advised of any changes in water quality.

Facts about cyanobacteria:

• Algal blooms appear as dark brown/black mats attached to rocks along the riverbed
•The algae occur naturally but can increase rapidly during warmer months
•It often has a strong musty smell and algal toxin concentrations can vary over short periods with changing environmental conditions
• Although high river levels will remove the algal bloom, detached mats can accumulate along the shore and increase the risk of exposure to toxins.
•If a health warning is in place avoid contact with the water
•Although district or city councils may place warning signs, these may not be seen at the numerous river access points, so people and dog-walkers should treat every low-flowing river cautiously.


For further information visit https://www.ecan.govt.nz/your-region/your-environment/water/swimming-water-quality/