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.
Phormidium toxic algae.
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.

Scroll down to see what to do if you spot toxic algae.

Current toxic algae alerts

Samples collected at the following sites show the presence of the toxic Cyanobacteria. Some cyanobacteria can produce toxins that are harmful to animals and humans. Please avoid the area until further notice and when it has been removed from the list below.

Cardrona River at the Confluence (11/03/23)


Useful information

For more information on potentially toxic algae, please visit the LAWA fact sheets below, watch the video and check out a list of sites across Otago where toxic algae has been sighted and confirmed.

Rivers: Known cyanobacteria hotspots in Otago

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


Lakes: Known cyanobacteria hotspots in Otago

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


What should I do if I suspect there's toxic algae?

If you see anything that you think could be a cyanobacteria bloom or phormidium, call the ORC Pollution Hotline on: 0800 800 033 (answered 7 days a week) or email us to let us know what you have observed.

Any photographs of suspected algal blooms that you can send us of will help us to quickly identify the type of algae and notify people of any risk that may be present.

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