| 10 May 2023

Health Warning lifted for Algal Bloom in the Ōpihi River at State Highway 1

Te Mana Ora | Community and Public Health has lifted its algal bloom health warning issued for the Ōpihi River at State Highway 1. This was issued on 8 February 2023.

Recent cyanobacteria surveys of the Ōpihi River has shown the cover of potentially toxic algae (benthic cyanobacteria) in the river has reduced and is now below levels that are of concern to public health.

Dr Cheryl Brunton, Medical Officer of Health for Te Mana Ora, National Public Health Service Te Whatu Ora, says Environment Canterbury’s monitoring of Ōpihi River will resume next summer when there is increased likelihood of cyanobacteria growth.

Warnings to avoid the following waterways due to ongoing algal blooms are still in place:

  • Saltwater Creek (Ōtipua) downstream of Centennial Park
  • Lake Opuha.
Facts about benthic 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 (e.g. 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.

For further details visit: https://www.lawa.org.nz/explore-data/canterbury-region/

Or contact Te Mana Ora on (03) 364 1777:


For more information about Mahinga Kai:


For further information, contact:

Karen Berry
Communications Manager
Te Whatu Ora South Canterbury  
021 139 7442