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Freshwater Toxic Algae

Freshwater Toxic Algae

Updated 9 November 2021 1:34pm

Toxic algae in our rivers are actually not algae at all, but cyanobacteria, which is commonly referred to as blue-green algae. Cyanobacteria are naturally present in all New Zealand waterways. There are lots of types of algae and cyanobacteria that grow in our waterways, so it's important to know which ones are harmful.

During hot dry weather, toxic algae blooms can form in rivers, making the swimming sites especially dangerous for people and dogs. These blooms last until there is a flushing event due to heavy rain.

Toxic algae is harmful to humans and dogs when ingested, a piece the size of a 50c coin is enough to kill a dog.

How to spot Toxic Algae

Toxic algae blooms appear differently in lakes and rivers. We recommend you follow these tips to spot it in each location:


  • Look for black, green or brown slime on rocks, or brown or black "mats" at the river's edge that have a velvety texture and earthy/musty smell.
  • If you see toxic algae, be cautious and avoid that river site, particularly if you have a dog.
  • Check for alerts on the LAWA website, which provides live updates on where it is safe to swim.


Lakes in the Wellington region are not part of our monitoring program, as river swimming spots are much more popular. However, we encourage you to know what to look for in lakes as well as rivers.

  • If the water has a "pea soup appearance", it could contain toxic algae. Discoloured, cloudy water with small green blobs suspended in it should be avoided.


  • Toxic algae is quickly deactivated by saltwater, so is not harmful once it reaches the sea.
  • Even during severe toxic algal blooms this is not harmful in the sea. However, you should ensure the area is safe for swimming by visiting the LAWA website, as the sea can contain harmful bacteria.

Get in touch with the Greater Wellington Contact Centre on 0800 496 734 if you have spotted toxic algae in our region. It helps to take a photo and note your location before getting in touch.

Safety precautions


Dogs are most at risk as they like the smell and taste of dried toxic algae, they are most susceptible when mats wash up at the river edges.

If there has been an alert issued, or you think you have spotted a toxic algal bloom:

  • Keep your dog on a lead
  • Keep your dog out of the water
  • Ensure it does not eat any algal mats

If you are concerned about your dog, take it to the nearest vet immediately. Toxic algae can affect dogs within minutes in extreme cases. Make sure to tell your vet that you think it may have ingested toxic algae, so that they can give it the best treatment as quickly as possible.


Swallowing water containing toxic algae can make humans very sick with symptoms such as nausea, vomiting and diarrhoea. Contact can also cause irritation of the skin, eyes, nose and mouth.

Because children are inquisitive, they are more likely to pick up/touch toxic algae and then put fingers in mouths so special care should be taken when swimming with them.

If you think you, or your child, are experiencing a reaction after swimming/playing in a river, 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.

How we're helping

Greater Wellington works with other councils and Regional Public Health to monitor the safety of our waterways, and issue warnings when blooms occur.

This includes signs at key sites where toxic algae occurs, and updates online. However, people are advised to learn what toxic algae looks like, and swim elsewhere if they see it.

Head to the LAWA website to check for current warnings.