Health Warning: Harmful Algal Bloom in Waikirikiri-Selwyn River
Health NZ media release: 10th December 2025
Health New Zealand has issued a health warning for Waikirikiri-Selwyn River at Glentunnel and at Whitecliffs Road. The warning follows finding moderate to high cover of potentially toxic harmful algae (benthic cyanobacteria) at these locations.
People should avoid the area and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.
There are also other access points along the Waikirikiri-Selwyn River that may have benthic cyanobacteria present. People are advised to treat every low-flowing river cautiously, check for the presence of benthic cyanobacteria and avoid contact.
Medical Officer of Health Dr Annabel Begg says the algae 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," she says.
“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," Dr Begg says.
Environment Canterbury monitors the site during summer, and the public will be advised of any changes in water quality that are of public health significance.
Visit the LAWA website for more information on recreational water quality in Canterbury, including keeping up to date with current health warnings.
Contact your local National Public Health Service office.
Facts about cyanobacteria
- 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 in the water and sediments (such as nitrogen and phosphorus), a sustained period of low and stable flows, and favourable weather conditions (such as increased temperature and calm days).
- It often has a strong musty smell and algal toxin concentrations can vary over short periods.
- 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, hence the need for people and dog-walkers to treat every low-flowing river cautiously.
- Pets that show signs of illness after coming into contact with algal mats should be taken to a vet immediately.