Cyanobacteria Toxic Algae
Cyanobacteria, commonly known as toxic algae, are naturally found in many of Aotearoa New Zealand’s rivers, lakes, and coastal waters. They are usually present at low levels and harmless, but under certain conditions they can form blooms or thick mats. Some species can produce toxins that are harmful and potentially fatal ifswallowed.
In rivers, cyanobacteria often grow as dark mats on the riverbed, margins, and shallow areas, and may break free and collect downstream. In lakes and coastal waters, cyanobacteria can also occur as floating blooms, particularly during warmer months.
If you're swimming, check out LAWA's Can I Swim Here?
Please note that not all rivers are monitored.
When and Why Toxic Algae Grows
Toxic algae blooms are more likely to occur when:
- river flows are slow or stagnant
- temperatures are warm
- conditions remain stable for extended periods
This means blooms often develop during summer and early autumn, particularly in clean rivers with low flows. Floating (planktonic) cyanobacteria can be present year‑round in lakes and coastal waters but usually increase when temperatures rise.
Health Risks
Cyanobacteria toxins can pose serious health risks if ingested. People, children, dogs, and livestock are all potentially affected.
Exposure can occur through:
- swallowing contaminated water
- contact with algal mats or scums
- dogs licking algae from their fur or paws
Dogs are particularly vulnerable and can become seriously ill very quickly.
Monitoring and Alert Levels
Council monitors cyanobacteria at selected river sites during the warmer months. Not all rivers are monitored, so it’s important to stay alert, check conditions, and look for warning signage when visiting waterways.
Alert Levels
Alert levels are based on the percentage of riverbed covered by cyanobacteria mats at monitoring sites. Risk may also increase when algae mats break away upstream and collect downstream.
| Alert Level | Health Risk | Cyanobacteria Coverage | What This Means |
|---|---|---|---|
| Low (Green) | Low | 0–20% | Most river areas are free of toxic algae. Take usual care and stay aware. |
| Medium (Amber) | Low–Moderate | 20–50% | Toxic algae has increased. Keep children and dogs away from the water and river margins. |
| High (Red) | High | 50%+ | Toxic algae is widespread. Do not swim. Keep children, dogs, and animals away from the river and margins. |
Always observe any warning signs placed at river access points.
See Table: Toxic algae monitoring Phormidium cyanobacteria % cover at river monitoring sites (note: < = less than)
| Date | Maitai at Avon Terrace | Maitai at Dennes Hole | Maitai at Campground | Wakapuaka at Hira Reserve | Isel Park | ||
|---|---|---|---|---|---|---|---|
2026 | |||||||
| 11 Feb | 1.5% | <1% | 2.5% | 3.75% | 5% | ||
| 30 Jan | 3.5% | <1% | 1.25% | 5.5% | 3.25% | ||
| 16 Jan | 2% | <1% | 2.25% | 6.25% | 2% | ||
| 9 Jan | 2.5% | <1% | 1.5% | 6.75% | 1.25% | ||
| 2025 | |||||||
| 31 Dec | 3% | <1% | 1.75% | 7.5% | <1% | ||
| 24 Dec | 4.25% | 1% | 1.75% | 6% | <1% | ||
| 19 Dec | 2.5% | 1% | <1% | 6.25% | <1% | ||
| 11 Dec | 10.95% | 2.5% | 10% | 7.75% | <1% | ||
| 5 Dec | 11% | 2.25% | 10% | 4.25% | <1% | ||
| 26 Nov | 15% | 1.75% | 7.5% | 5.75% | 0% | ||
| 7 Mar | 0% | 0% | 0.25% | 0% | 5.5% | ||
| 21 Feb | 0.5% | 0% | 0% | 0% | 1.5% | ||
| 5 Feb | 3% | 0.25% | 2.5% | 2.5% | 3.75% | ||
| 23 Jan | 1.75% | 0.25% | 0% | 6.75% | 5.5% | ||
| 10 Jan | 1.5% | 0.5% | 1.75% | 3% | 2% | ||
Swimming and Recreation
Before swimming, always check LAWA’s “Can I Swim Here?” for the latest water‑quality information and health alerts.
Even at sites that usually have good water quality, it is not recommended to swim during toxic algae alerts.
Avoid Swimming After Heavy Rain
Heavy rain can flush contaminants from urban and rural land into waterways and may also move algae mats downstream.
As a precaution:
- avoid swimming for at least 24–72 hours after heavy or prolonged rain, especially if the water appears cloudy, discoloured, or has visible algae
- always check the water is clean and clear before entering
What Does Toxic Algae Look Like?
Cyanobacteria can appear in different forms depending on the waterway and conditions. It often looks like dark mats on rocks or river margins, or green or brown scums in the water. The examples below show common ways toxic algae may appear in Nelson’s rivers.
If you see algae like this, do not touch it and keep people, dogs, and livestock away from the water.
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 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, and the results are published on the LAWA Can I Swim Here module. There is permanent signage at swimming holes, and Facebook posts will update the public as needed.
