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Freshwater Toxic Algae

Freshwater Toxic Algae

Updated 4 November 2020 3:32pm

Freshwater toxic algae, known scientifically as cyanobacteria, are an ancient group of photosynthetic bacteria that are common in freshwater in New Zealand. There are multiple different species of toxic algae some of which can cause blooms in lakes and rivers across the country.

Toxic algal blooms can occur in rivers and streams that are generally considered to have good water quality and don't cause any problems most of the time. During summer, with higher temperatures and long dry periods, cyanobacteria can form extensive blooms which can be toxic, particularly to dogs if they eat the algal mats. 

Cyanobacteria are not always toxic. The only way of working out if they are toxic is laboratory testing. It is our job to monitor and test and keep you informed if there are issues.

Check for alerts

View our interactive map to see if there are any toxic algae warnings currently in place

Know what to look for

We monitor toxic algae at popular river spots around the region. However, it’s important to know what to look for and check before getting in the water.  

In rivers:

  • Toxic algae generally form brown, dark green or black mats that grow on rocks in the river bed. Mats can come loose from the riverbed and wash up on the banks or form floating ‘rafts’ in shallow areas.
  • When exposed, the mats may dry out and turn light brown. 
  • The mats produce a strong musty odour. Dogs are attracted to the smell, and may eat any accessible mats that wash up on the river’s edge.

Cyanobacteria differ from harmless bright green algae, which often form long filaments. It also differs from ‘diatom’ mats, which tend to form white/brown mats that appear rougher in texture than the smooth velvety toxic algal (cyanobacteria) mats.

Toxic algal mat growing on the river bed Toxic algal mat growing on the river bed

In lakes and slow-flowing waters:

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.

  • Cyanobacteria grow in a free-floating (also called planktonic) form which can cause the water to become murky or cloudy. Free-floating cyanobacterial blooms are generally green in colour and can give lakes a ‘pea soup’ appearance.
  • Free-floating cyanobacteria can also form films or scums on the water’s surface, especially at the water’s edge. 

Henley Lake in Masterton, Whitby Lakes in Whitby, and Lake Waitawa north of Ōtaki often have high levels of toxic algae. We recommend you don't let your dog swim in or drink from these lakes.  Henley Lake is monitored by Masterton District Council during summer. 

Planktonic cyanobacteria bloom (Photo: Kaine Jacquiery, Masterton District Council) Planktonic cyanobacteria bloom washed up on the edge of Lake Waitawa

Enjoying our rivers

Toxic algae can be harmful, but if you check for alerts and know what to look for, you can still enjoy the water safely during summer. Just like you wouldn’t swim at the beach if you spot a rip, you shouldn’t swim in a river while there is a toxic algal bloom.

To be safe this summer, you should always check for alerts before you head to the river, and avoid any contact with toxic algae. Although you usually have to eat the algae for it to be harmful, some people can suffer skin irritation after coming into contact with it. If you experience a reaction after contact or swimming, contact your doctor.

Keeping your dog safe

Dogs are most at risk as they like the smell and taste of 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.

To keep your dog safe, check for alerts on the LAWA 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. 

If you suspect that your dog has eaten toxic algae, contact your vet immediately. In extreme cases, death can occur within 30 minutes after the first signs of illness appear. Signs a dog has been poisoned by toxic algae include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.

Our monitoring programme

We monitor toxic algae at popular river spots around the region. Monitoring is routinely undertaken at high priority sites during the summer (usually between mid-November to the end of March, and extended if required).  If toxic algae is on the rise, or if local conditions are known to exacerbate the risk of toxic algae blooms, then the routine monitoring is increased and extended to other popular sites.

The risk to people and dogs from toxic algae is determined by measuring the proportion of the river bed it covers as well as the detached mats washed up on the river’s edge. Results are compared to national guidelines which provide trigger values for three levels of risk from toxic algae – surveillance, alert and action.

Alert level Trigger level Management response
Surveillance ≤20% coverage of potentially toxic algae attached to substrate. Undertake routine monitoring.
Alert 20–50% coverage of potentially toxic algae attached to substrate. Notify public health, erect signs with information on appearance of mats and potential risks and consider toxin testing.
Action >50% toxic algae coverage or toxic algae are visibly detaching from substrate and accumulating on the river’s edge or becoming exposed on river’s edge and the river level drops. Notify public health unit, notify the public of potential risk to health, and consider toxin testing.

In the Wellington Region, the response to toxic algal blooms in rivers is managed jointly with Regional Public Health and your local council.

Read the national guidelines published by the Ministry for the Environment.


Henley Lake in Masterton is monitored for cyanobacteria blooms by Masterton District Council. Check their website and press releases for updates on the algae levels in the lake.

Most other lakes in the region are not routinely monitored for cyanobacteria blooms. However, warning signs may be put in place when an issue is detected.