Health New Zealand has issued a health warning for the Ashley River at SH1 after detecting moderate to high levels of potentially toxic algae (benthic cyanobacteria).

The public is advised to avoid the area, and animals, especially dogs, should be kept away from the water until the warning is lifted. Other access points along the river may also have toxic algae present, and people are urged to treat all low-flowing rivers with caution, checking for signs of algae and avoiding contact.

Public Health Medicine Specialist Dr Annabel Begg said the algae appear as dark brown to black mats and can produce toxins harmful to both humans and animals.

“Exposure may cause skin rashes, nausea, stomach cramps, tingling, and numbness around the mouth and fingertips,” Dr Begg said.

Anyone experiencing these symptoms after contact with algal mats or water from the area should seek urgent medical advice and inform their healthcare provider about the exposure. Pets showing signs of illness should be taken to a vet immediately.

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

What you need to know about cyanobacteria:

•It looks like dark brown or black mats attached to rocks along the riverbed.

•The algae naturally occur in low levels but can grow rapidly in warmer months, particularly in low-flowing rivers with increased nutrients, stable flows, and favourable weather.

•It often has a strong musty smell, and toxin levels can change quickly.

•High river flows may clear the algae, but detached mats can wash ashore, posing a risk of exposure.

•Even without warning signs, people should avoid contact with water where cyanobacteria may be present.

The warning remains in place until further notice, and the public is advised to follow updates from Environment Canterbury.