Frequently Asked Questions

What is toxic algae (cyanobacteria)?

Toxic algae (also known as Cyanobacteria and blue-green algae because of the cyano colour of cells) are true bacteria that live like algae and are naturally present in many New Zealand waterways. Toxic algae are found in ‘clean’ rivers and less likely to be present in high nutrient load waters where filamentous algae grow. The growth of toxic algae is encouraged when river flows are low and stable, and temperatures are consistently warm.

What do the mats look like?

toxic algae tim cuff Toxic algae mats are actually dark brown or black and grow attached to rocks on the river bed. Mats that come loose from the river bed can wash up on the river bank or form floating ‘rafts’ in shallow areas. Where exposed, the mats may dry out and turn a light brown colour. They also produce a strong musty odour, which may attract dogs to eat the mat.

What is the problem with toxic algae?

Toxic algae mats can produce toxins that are harmful to humans, dogs, livestock and wildlife when ingested. However, toxins are not produced by mats all the time.

What are the possible health effects?

Swallowing water containing toxic algae toxins can lead to vomiting, diarrhoea, abdominal pain, cramps, nausea and other effects in humans. Skin contact can cause irritation of the skin, eyes, nose and mouth. 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. Livestock are also at risk from poisoning

What should I do if I find toxic algae?

Rivers users, particularly those with dogs, or those taking water for livestock or human consumption, should avoid contact with any thick dark brown-black algal mats, particularly those that are easily accessible, exposed on river edges or floating in shallow areas of riverbanks or near rocks. If toxic algae mats are widespread in a river you should presume that the water may be unsafe for bathing or drinking.

Who should I call if I experience a reaction?

If you think you are experiencing a serious reaction, seek urgent medical attention. Advise your doctor of your potential exposure to toxic algae. Your GP has been asked to notify Public Health of any people with possible reactions.

Who should I call if I think my animal is sick?

If you are concerned about your animal(s), contact a vet immediately (the toxins can affect dogs within minutes). You or your vet can report any animal illness resulting from contact with toxic algae to Council.

What is Council doing about it?

Council is working with the NMDHB Health Protection Team and Cawthron, taking a proactive approach with weekly monitoring at key recreation sites in the Maitai River. There is permanent signage at swimming holes and media releases will update the public as needed.

algae warning signs both
Additional research is underway as part of the Project Maitai/Mahitahi programme to improve the river’s health, which will involve Cawthron scientists and students assessing the key reasons for toxic algae blooms and providing recommendations on how to manage and predict where nuisance blooms may occur. If you see cyanobacteria mats, please contact the Council on 546 0200

Don't swim after heavy rain

Contamination of river water can occur during or shortly after rainfall from storm water runoff. To minimise the risk of exposure to harmful bacteria it is best not to swim for the first 24 hours at least after rainfall that leads to any discolouration of the water.

Download an information sheet including photos (589KB PDF).