Toxic algae FAQs

Toxic algae FAQs

What are toxic algae?

How can I keep myself and my animals safe from toxic algae?

How is toxic algae monitored?

Click here to see current toxic algae warnings 

What are toxic algae? 

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.  

Rivers and streams: What does mat-forming cyanobacteria look like?



Toxic algal mat growing on the river bed

Toxic algal mats washed up on the river's edge

Lakes and ponds: What does planktonic cyanobacteria look like?

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.

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)

What causes cyanobacteria blooms?

Like other types of algae, cyanobacteria growth increases during times of warm temperatures, sunlight and low or stable river flows.  For this reason blooms generally occur in summer months.  Nutrients such as nitrogen and phosphorus can also contribute to cyanobacteria blooms.

Are they always toxic?

No. Not all species produce toxins and even those that do don’t produce toxins all of the time.  The presence of toxins can only be identified by laboratory testing.  If you are unsure whether cyanobacteria are toxic or not, it is safest to assume they are. 


Avoid contact with cyanobacteria in both lakes and rivers. If cyanobacteria are widespread you should presume that the water is unsafe for swimming or drinking – this includes taking water for livestock consumption. Keep your dog out of the water and most importantly, ensure it does not eat any algal material in the water or at the water’s edge.

Who should I call if I experience a reaction?

In humans, skin contact with the cyanobacteria can cause irritation of the skin, eyes, nose and mouth, and if swallowed, can cause vomiting, diarrhoea, abdominal pain, cramps and nausea. If you think you are experiencing a reaction due to contact with toxic algae, seek urgent medical attention. Advise your doctor of your potential exposure to cyanobacteria and ask them to notify Regional Public Health

Who should I call if or my dog is sick?

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.

Is it safe to drink water containing toxin-producing cyanobacteria?





If cyanobacteria are widespread in a river or lake you should assume it is unsafe for swimming. You should avoid any skin contact with the water and avoid swallowing the water. The higher the concentration of cyanobacteria and the longer the time spent in the water, the more severe the symptoms are likely to be.  Wearing a wetsuit will not protect you and may cause severe irritation around the collar and cuffs.


Eating mussels and other shellfish from affected areas should be avoided as they can concentrate the toxins produced by cyanobacteria. Fish from waters containing toxic cya­nobacteria may be eaten in moderation. Avoid eating the liver and kidney of the fish, as this is where the accumulation of toxins may be greatest. Fish may taste earthy. Avoid contact with the water while fishing and wash all fish in clean water.


How safe boating and canoeing are depends on the amount of direct contact with the water and the concentration of cyanobacteria. If you swallow the water or your skin is in contact with the water while boating or canoeing, you are at risk from a reaction to any toxins that may be present. The higher the concentrations of cyanobacteria and the longer that people are in contact with the water, the more likely a reaction is to occur. Wash boats or canoes and life-jackets down with clean water after use.





Where can I get more information?


How is toxic algae monitored? 


Greater Wellington Regional Council monitors toxic algae growth at 20 popular river swimming spots around the region.  Click here to see which sites are monitored. Monitoring is undertaken weekly during the official summer swimming season (mid-November 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.

Surveillance, alert and action trigger levels for cyanobacteria in rivers as specified by national guidelines

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 by a working party of Regional Public Health, Greater Wellington Regional Council, city and district council staff.

Click here to see whether there are any toxic algae warnings in place.


Henley Lake in Masterton is monitored for cyanobacteria blooms by Masterton District CouncilClick here for more information.

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.

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