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Toxic algae FAQs

Toxic algae FAQs

Updated 3 December 2019 12:11pm

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Toxic Algae

Freshwater toxic algae, known scientifically as cyanobacteria, are an ancient group of photosynthetic bacteria that are common in rivers and lakes in New Zealand. They occur in rivers and streams that are generally considered to have good water quality and most of the time don't cause any problems. During summer, with higher temperatures and lower water levels, 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. You can check your favourite swimming spot on our map for alerts:

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

Keep an eye out for toxic algae

In rivers, toxic algae generally form brown, dark green or black mats that grow on rocks in the river bed. Mats that come loose from the riverbed can wash up on the banks or form floating ‘rafts’ in shallow areas. When exposed, the mats may dry out and turn a light brown or white colour and may also produce a strong musty odour. Dogs love the smell and can eat it. Even a small amount, the size of a 50 cent piece, can kill a dog. 

Cyanobacteria differ from harmless bright green algae, which often form long filaments.

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

In lakes and slow-flowing waters, 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 and Lake Waitawa (North of Ōtaki) often have high levels of toxic algae and we recommend you don't let your dog swim in or drink from these lakes.

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

Enjoying our rivers and lakes

Check for alerts before you head to the river or lake. Avoid contact with toxic algae but you do have to eat the algae for it to be harmful. A small number of people are very sensitive and may experience skin irritation after being exposed. If you experience a reaction after contact or swimming, contact your doctor.

Dogs are most at risk as they like the smell and taste of toxic algae. Keep an eye on your dog when near the river or lake. If there is an alert or you think you have spotted toxic algae mats keep your dog on a lead, out of the water and most importantly, ensure it does not eat any algae mats in the water or at the water’s edge.

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 extended to other popular sites.

The risk to people and animals 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.

The Ministry for the Environment publishes the national guidelines and these are available on their website.


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.

No other lakes in the region are routinely monitored for cyanobacteria blooms.  However, warning signs may be put in place when an issue is detected.

 Toxic algae FAQs

What causes toxic algal blooms?

Toxic algae needs warm sunshine and low water flows to bloom.

Shallow gravel-bottomed rivers are easily warmed by stones absorbing the sunshine, and are most susceptible to blooms.

How long does a bloom last?

A toxic algal bloom will last until either the weather turns cold or a few days of heavy rain washes it away – this is called a “flushing event”.

What happens when the toxic algae reaches the sea – is it still poisonous?

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.

Is high take of water causing more toxic algal blooms?

Toxic algal blooms flourish and persist when water flows are low. Low flows occur naturally when there is little rain, however, research has shown no direct link between water take and toxic algae blooms. 

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

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

Water conservation to enable higher flows could also help – rural, industrial and domestic users can all play their part.

Is Greater Wellington doing anything to reduce toxic algal blooms?

There is no quick or obvious solution to the growing number and volume of toxic algal blooms, but improving water quality and quantity is most likely to mitigate them.

Greater Wellington has responded to the National Policy Statement for Freshwater Management by setting up Whaitua Committees to set limits for water quality and quantity. You can find out more about the collaborative policy development process with you and your community on our website.