Toxic algae sightings

Naturally occurring algae can be toxic and harmful to human and animal health. In Otago, we commonly see two types of potentially toxic algae. Phormidium, which looks like dark tar mats on riverbeds and riverbanks; and cyanobacteria in lakes, which is green and looks like pea soup. For more information on potentially toxic algae, please visit the LAWA fact sheets below:


Below is a list of sites across Otago where toxic algae has been sighted and confirmed.

Current toxic algae alerts – 2021/22 summer


 Known phormidium hotspots in Otago

  • Silver Stream (Mosgiel)
  • Cardrona River
  • Manuherekia River
  • Hāwea River


WARNING: Phormidium mats detach from the riverbed and can wash up on riverbanks. Its musty smell in particularly attractive to dogs and, if eaten, phormidium can result in severe poisoning or death. Anyone whose dog is showing signs they may have consumed Phormidium, such as lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsion, should be treated as an emergency and referred to a vet immediately.

Phormidium toxic algae.

Back to top
Online Maps & Data: