Toxic algae

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 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 a small amount (the size of a 50 cent piece) of 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.

Learn what to look for when visiting freshwater sites 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.

These mats can come loose and wash up on the edge of the rivers, or form ‘floating rafts’ in shallow areas. As they dry out they turn light brown or white and look like dried leaves or cowpats. They produce a strong musty smell, and this is when it poses the biggest risk to our dogs, as they love the smell and many dogs will try to eat it if they get the chance.

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 also 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

Toxic algae (cyanobacteria) are known to grow in many of our hill-fed and some alpine rivers and may pose a risk to humans and dogs. Please be cautious and ensure you check any river for toxic cyanobacteria before jumping in. These rivers are known to have toxic algae blooms at times: 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, Hae Hae Te Moana 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 toxic algae could be below the threshold for a public health warning - view current health warnings.

Can I swim here?

We monitor water quality at many of the region’s recreational swim sites between November and March. However, we cannot monitor all sites. 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