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Frequently asked Questions

Frequently asked Questions

Updated 4 November 2020 3:33pm

What is toxic algae?

Toxic algae in our rivers are actually not algae at all, but ‘cyanobacteria’, which is commonly referred to as blue-green algae. Cyanobacteria are an ancient group of organisms that were among the first life on earth, and were responsible for converting earth’s early atmosphere from an oxygen poor to an oxygen rich environment.

Cyanobacteria are naturally present in all New Zealand waterways. There are lots of types of algae that grow in our waterways, so it’s important to know which ones are harmful.

Toxic algae is harmful to humans and dogs when ingested.

What do toxic algae ‘mats’ look like?

Toxic algal mats are dark brown or black and grow attached to rocks on the river bed. Mats can come loose from the rocks, and either wash up on the river’s edge or form floating ‘rafts’ in the shallows.

The mats may dry out and turn a light brown colour. They also have a strong musty smell, which attracts dogs to eat them.

How can I tell if it’s present?


  • 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


  • 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
  • Greater Wellington Regional council does not monitor lakes in our region

Why do toxic algal blooms happen?

Toxic algae blooms tend to occur in warm and dry weather.

Shallow cobble-bottomed rivers are most susceptible, as the stones provide a stable platform for growth.

How long does a bloom last?

A toxic algal bloom will last until either it naturally sloughs from the riverbed or heavy rain washes it away, this is called a ‘flushing event’.

What should I do if I find it?

  1. Avoid the water for swimming or walking your dog
  2. Contact rivers users, particularly those with dogs, to inform them that you have found toxic algae in the water and that it might cause them or their dog to become sick
  3. Contact the Greater Wellington Contact Centre on 0800 496 734

Is it still toxic when it reaches the sea?

No – toxic algae is quickly dispersed and deactivated by salt water. Even during severe toxic algal blooms the sea is safe to swim in, and you can safely walk your dog on the beach.

You should ensure the area is safe to swim by visiting the “Can I swim here” page on the LAWA website, as the sea can contain harmful bacteria, particularly after heavy rain. A good rule of thumb is to wait 48 hours after rain before getting in the water.

How do I know whether it’s safe to swim before I go?

You can check for any known issues in the waterways in our region by visiting the “Can I swim here” page on the LAWA website.

Algal blooms can happen quickly, so we encourage people to know what toxic algae looks like as well as checking for alerts. With this information, they can spot potential blooms that may not have been picked up by monitoring and make informed decisions.

What are the possible health effects for humans?

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.

Children: 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.

What do I do if I think my child has swallowed some?

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. Your GP will have been asked to notify Public Health of any people with possible reactions.

What do I do if my dog has swallowed some?

If you are concerned about your dog, take it to the nearest vet immediately. Toxic algae can affect dogs within minutes in some 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.

How do I keep my dog safe?

Dogs are most at risk as they like the smell and taste of dried toxic algae. A small amount, the size of a 50 cent piece, can be enough to kill a dog. Dogs are most susceptible when mats wash up at the river edges.

Check for alerts on the Greater Wellington website before you go to a river, look for warning signs, and keep an eye on your dog when you’re there.

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.

What are local councils doing about it?

There is no quick or obvious solution to prevent toxic algal blooms, but improving water quality and quantity is most likely to help reduce their frequency and magnitude.

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 inform the Greater Wellington Contact Centre (0800 496 734) if they see it.

How do toxic algae affect the environment?

Dense blooms of toxic algae can be dangerous to aquatic life. Algae can block sunlight and smother the riverbed, which affects other animals such as macroinvertebrates and fish.

Can I do anything to reduce the likelihood and severity of toxic algal blooms?

We can lower the amount of nutrients and sediment entering into waterways by excluding livestock, using less fertiliser (urban and rural), fixing sewage leaks and ensuring septic tanks are well maintained.