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. Cyanobacteria in rivers, 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 

  • 15 November 2022:
    Butchers Dam
    Samples collected at Butchers Dam show the presence of the toxic Cyanobacteria. Please keep your dogs on a leash to steer well away from the water, and stay out of the water yourself.


Rivers: Known cyanobacteria hotspots in Otago

  • Silver Stream, Mosgiel
  • Cardrona River, Central Otago
  • Manuherekia River, Central Otago
  • Hāwea River, Central Otago
  • Waianakarua Rover, North Otago


Lakes: Known cyanobacteria hotspots in Otago

  • Lake Waihola, Dunedin
  • Tomahawk Lagoon, Dunedin
  • Butchers Dam, Central Otago


WARNING: Cyanobacteria in rivers form mats that detach from the riverbed and can wash up on riverbanks. Its musty smell in particularly attractive to dogs and, if eaten, can result in severe poisoning or death. Anyone whose dog is showing signs they may have consumed cyanobacteria mats, 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.

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