Health Warning lifted for Harmful Algal Bloom in Ōpihi River at State Highway 1 bridge
South Canterbury
Day Lifted
Tuesday 10 February 2026
Day Issued
Monday 12 January 2026
Health New Zealand | Te Whatu Ora has lifted its harmful algal bloom health warning issued for the Ōpihi River at State Highway 1 bridge.
Recent cyanobacteria surveys of the Ōpihi River at the State Highway 1 bridge has shown the cover of potentially toxic harmful algae (benthic cyanobacteria) has reduced and is now below levels that are of concern to public health.
Dr Ramon Pink, Medical Officer of Health for the National Public Health Service, says Environment Canterbury’s monitoring of Ōpihi River will continue to the end of this summer and then will resume next summer when there is increased likelihood of cyanobacteria growth.
Facts about cyanobacteria
- Cyanobacteria appears as dark brown or 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 (for example as increased temperature, 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/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.