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Toxic algae

Toxic algae

Updated 9 January 2018 10:05am

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 waterways with 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 algae mats. 

Cyanobacteria are not always toxic. The only way of working out if they 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, cyanobacteria generally form brown 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 mats washed up on the river's edg

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 often has high levels of toxic algae and we recommend you don't let your dog swim in or drink from the lake.

Planktonic cyanobacteria bloom (Photo: Kaine Jacquiery, Masterton District Council)



Planktonic cyanobacteria bloom washed up on the edge of a lake (Photo: Susie Wood, Cawthron Institute)

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.

It is the furry member(s) of your family that are at the most risk. Keep an eye on your dog when they are 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, you should treat it like an emergency and 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 a number of popular river spots around the region. Monitoring is undertaken weekly during the summer (beginning of December to the end of March).

The risk to people and animals from toxic algae is determined by measuring the proportion of the river bed covered by cyanobacteria as well as the amount of cyanobacteria 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 cyanobacteria attached to substrate. Undertake routine monitoring.
Alert 20–50% coverage of potentially toxic cyanobacteria attached to substrate. Notify public health, erect signs with information on appearance of mats and potential risks and consider toxin testing.
Action >50% cyanobacteria coverage or cyanobacteria 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.

Freshwater toxic algae, known scientifically as cyanobacteria, are an ancient group of photosynthetic bacteria. Cyanobacteria are widespread in rivers and lakes in New Zealand including waterways with good water quality. Under certain environmental conditions cyanobacteria can form extensive blooms which can be toxic to humans, dogs, livestock and wildlife. Exposure to high levels of toxins can result in serious illness or death. Dogs are particularly susceptible to poisoning from cyanobacteria as they love to scavenge and play near water. Toxic algal blooms have been recorded in a number of rivers and lakes around the region and across New Zealand.