Naturally occurring toxic algae (or cyanobacteria) can be harmful to human and animal health.

In Otago, we commonly see toxic algae blooms in lakes and rivers during the summertime. It may be present at sites not listed here – conditions can change quickly and so can the risk of contact.

The best way to stay safe is to learn how to spot toxic algae. Always check the area for signs of toxic algae before you (and your pets) play in or around the water.

Current toxic algae alerts

There are currently no alerts.

Toxic algae surveillance monitoring for the 2023-24 summer bathing season has now ended. For more information about how to identify toxic algae see our Toxic Algae Information page.


Toxic algae in lake looks like green pea soup.



Toxic algae in rivers looks like dark tar mats on riverbeds and banks.


Known cyanobacteria hotspots in Otago


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


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

Report suspected toxic algae blooms

If you think you have experienced a reaction after exposure to water containing toxic algae, seek urgent medical attention. Let your doctor know that you think you have swallowed toxic algae, so that they can inform Regional Public Health of the incident.  

Swallowing water containing toxic algae lead to vomiting, diarrhoea, abdominal pain, and nausea. Skin contact can cause irritation of the skin, eyes, nose, and mouth. The toxins can also affect the liver and the nervous system.

Dogs are particularly susceptible to poisoning as they can be attracted to the musty smell of toxic algae mats – if eaten it can cause severe poisoning or death. In extreme cases death can occur within 30 minutes after signs first appear. If your dog experiences these symptoms treat it as an emergency and contact a veterinarian immediately.

Livestock are also at risk from poisoning from toxic algae and should be provided with alternative drinking water.

Symptoms of poisoning in animals include:

  • Lethargy
  • Muscle tremors
  • Fast breathing
  • Twitching
  • Paralysis
  • Convulsions

See our Toxic Algae information page to learn more about toxic algae.

View the latest water quality results for popular swimming spots and useful factsheets at LAWA


What should I do if I see toxic algae?

Report suspected toxic algal blooms to the ORC Pollution Hotline 0800 800 033 (7 days) or email us 

Photographs of observed algal blooms will help us to quickly identify toxic algae and notify others about potential risks.