Health New Zealand has issued a health warning for the Ashley River at the Gorge Bridge following the discovery of moderate to high levels of potentially toxic algae, known as benthic cyanobacteria.

Residents and visitors are being urged to avoid the area, and pet owners are advised to keep animals, particularly dogs, away from the water until the warning is lifted.

Other access points along the Ashley River near the Gorge Bridge may also be affected, and people are advised to approach all low-flowing rivers with caution.

Dr Cheryl Brunton, Medical Officer of Health for the National Public Health Service, said the algae, which appears as dark brown or black mats attached to rocks, can produce harmful toxins.

“Exposure may cause skin rashes, nausea, stomach cramps, tingling, and numbness around the mouth and fingertips,” said Dr Brunton. “If you experience any of these symptoms, seek medical advice urgently and let your healthcare provider know you’ve had contact with dark brown or black algal mats or water in this area.”

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

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

What to know about cyanobacteria:

•Appears as dark brown or black mats attached to rocks along riverbeds.

•Naturally occurring, but blooms can escalate during warmer months with low water flow, increased nutrients (like nitrogen and phosphorus), and stable weather conditions.

•Produces a strong musty smell, and toxin levels can fluctuate rapidly.

•High river levels can dislodge algae mats, which may accumulate along the shore, increasing exposure risks.

While some areas may have warning signs posted, Health New Zealand reminds people that signs may not cover all river access points. Dog walkers and visitors are advised to remain vigilant and treat all low-flowing rivers with caution