Health New Zealand has issued a health warning for Selwyn River at Glentunnel and Whitecliffs following the discovery of moderate to high levels of potentially toxic algae (benthic cyanobacteria).

Water testing results have shown that faecal bacteria levels at the Glentunnel site are now below guideline values after a warning was issued on 2 January 2025. However, the presence of benthic cyanobacteria at the site means the health warning remains in effect.

The public is being urged to avoid the area, and animals, particularly dogs, should not be allowed near the water until the warning is lifted. Other access points along the Selwyn River may also contain benthic cyanobacteria, and people are advised to treat all low-flowing rivers cautiously, check for algae, and avoid contact.

Dr Annabel Begg, Public Health Medicine Specialist for the National Public Health Service, said the algae appear as 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,” Dr Begg said.

“If you experience any of these symptoms, seek medical advice urgently and let your healthcare provider know you’ve had contact with dark brown/black algal mats or water in this area.”

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

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

Facts about cyanobacteria:

•Appears as dark brown/black mats attached to rocks along the riverbed.

•A low cover of the algae can occur naturally but can increase rapidly during warmer months. Algal blooms are influenced by a combination of available nutrients (such as nitrogen and phosphorus), sustained periods of low and stable flows, and warm weather conditions.

•It often has a strong musty smell, and algal toxin concentrations can vary over short periods.

•High river levels can remove algal blooms, but detached mats may accumulate along the shore and increase the risk of exposure to toxins.

•If a health warning is in place, avoid contact with the water.

•Warning signs may not be present at all access points, so treat every low-flowing river cautiously, especially when walking dogs.