Toxic algae

Dog in a river with the text 'If in doubt, keep them out' Potentially toxic algae (also called cyanobacteria) is a naturally occurring algae with the potential to bloom, producing toxins that can harm or even kill dogs, people and other animals. 

Unfortunately, dogs love the musty smell of potentially toxic algae and are naturally drawn to investigating it – usually by licking or ingesting the material when out enjoying rivers and lakes across the country.

Even if a small amount (the size of a 50 cent piece) of potentially toxic algae can be harmful if ingested.

Water containing toxic algae can pose a risk to health through recreational contact, drinking and gathering shellfish and mahinga kai resources.

Knowing what to look for when visiting Canterbury’s freshwater sites is the best way to keep your whānau and dogs safe.

Toxic algae in rivers: what you need to know

What toxic algae looks like in rivers

This cyanobacteria (called benthic cyanobacteria) grows on the bottom of river beds. It appears as thick dark brown or black mats that have a slimy or velvety texture and musty smell.

Cyanobacteria in riverbed
Cyanobacteria on rocks

What toxic algae looks like in lakes, ponds and lagoons

Planktonic cyanobacteria (floating algae) is often suspended in the water. When it blooms, it can make water look cloudy, discoloured, or like it has small globules in it. There may not be obvious visual traits.

Cyanobacteria in lake
Cyanobacteria in pond

What causes toxic algae

Blooms form when cyanobacteria start to multiply very quickly, usually due to changes in environmental conditions such as warm temperatures, sunlight, low or stable river flows, and high levels of nutrients.

Know the symptoms of poisoning

For people

  • tingling or numbness around the fingertips and/or mouth
  • breathing difficulties
  • gastrointestinal symptoms
  • skin rashes.

For dogs and livestock

If your dog is showing these symptoms after being in contact with a waterway, contact a veterinarian immediately. You or your vet can report any animal illness resulting from contact with cyanobacteria to us.

  • panting
  • lethargy
  • muscle tremors
  • twitching
  • convulsions.

In extreme cases, death can occur 30 minutes after symptoms first appear.

Caution advised at these rivers in Waitaha/Canterbury

Potentially toxic algae (cyanobacteria) are known to grow in these rivers and may pose a risk to dogs - please be cautious and ensure you check the area for potentially toxic cyanobacteria when visiting: Ōtākaro/Avon River, Rakahuri / Ashley River, Waikirikiri / Selwyn River, Opihi River, Otaio River, Pareora River, Temuka River, Waiau River. Waipara River, Waihao River, Waihi River, Waimakariri River, Leader River and Waitohi River.

Monitored swimming spots along these rivers may not have an active public health warning. This is because the amount of potentially toxic algae is below the threshold for a public health warning - view current health warnings.

LAWA Can I Swim Here

We monitor water quality at some of the region’s recreational swim sites between November and March. However, we cannot monitor all sites and water quality results on Land Air Water Aotearoa (LAWA) and health warnings are based on the risk to human health, not animals.

Find out more