Algal blooms

On this page:

Algal blooms and their effects •
Water quality monitoring and results •
Avoid contact •
Public health alerts and warnings •
Health or drinking water concerns

Algae occur naturally in our rivers, lakes, streams and coastal waters, and flourish during hot and fine conditions. Cooler temperatures make blooms less likely for most waterways, but winter blooms have been documented in Aotearoa New Zealand since 1970/80s.

Most algae are harmless, but high levels of some species may be toxic, and blooms may result in health warnings being issued.

During the warmer months, Waikato Regional Council and partner organisations monitor lakes and the Waikato River (including the hydro lakes). Some of the region's lakes are also monitored during winter months.

Algal blooms and their potential effects

Cyanobacteria or blue-green algae are often found in waterbodies throughout New Zealand.

Cyanobacteria are also called toxic algae as they have the potential to produce toxins that can affect public health. Contact with these toxins can also cause skin irritations and other adverse health effects, such as asthma attacks, rapid onset of nausea and diarrhoea. Extra care should be taken with toddlers and dogs as they like to put things into their mouths and even a small amount (the size of a 50-cent piece) can be enough to cause serious harm if eaten.

Blue-green algal bloom near Lake Ohakuri
Marine algal blooms are occurrences of rapid and excessive growth of algae in marine environments and may consist of both microscopic organisms (phytoplankton) and larger macroalgae such as seaweeds.

Marine algae are a natural and important part of the marine ecosystem and form the base of marine food webs which other organisms rely upon.

When environmental conditions are suited to marine algal growth, they may sometimes form large “blooms” leading to the water appearing green, orange, red, or the excessive growth of larger macro algae.

These blooms are a natural process and can be driven by factors such as changes in oceanic currents (upwelling), variation in nutrient inputs from land and riverine sources, increased sunlight, and warmer water temperatures.

However, sometimes certain algal species may produce toxins which can accumulate in marine organisms, potentially leading to seafood contamination and can pose health risks to humans and animals. Larger blooms can clog fish gills, wash up on beaches, and potentially lead to fish kills through de-oxygenation as the bloom dies off. Monitoring in coastal areas for shellfish exposure to paralytic shellfish poisoning (PSP), is undertaken by the Ministry of Primary Industries (MPI) and more information can be found on their website.

LAWA and MPI - Factsheets and information on algal blooms

Water quality monitoring and results

Waikato Regional Council and partner organisations monitor lakes and the Waikato River (including the hydro lakes) during the warmer months.

During winter, testing for blue-green algae is carried out in five lakes in our region (Waikare, Whangape, Waahi, Hakanoa, Ngāroto). For other lakes and rivers, it is carried out in response to reports about possible algal blooms in the region.

Up-to-date results are reported on the LAWA (Land, Air and Waikato Aotearoa) website.

LAWA provides this information so you can use it as a guide to decide where and when to swim, or use the water for other recreational activities, such as fishing, sailing or wading. Make sure you always check the LAWA site and their swim smart checklist before deciding whether the water is safe to use or enter.

Visit Lawa.org.nz/swim for up-to-date information and monitoring results

Please note that water quality monitoring results are representative at the time of sampling.
Heavy rain flushes contaminants from urban and rural land into waterways and we advise you not to swim for 2 – 3 days after heavy or prolonged rain – even at sites that generally have good water quality. Check that the water is clean and clear before taking a dip.

Cyanobacteria can rapidly bloom to harmful levels, and not all freshwater sites are monitored for these toxic algae.
Play it safe — if you can see cyanobacteria blooms in rivers or lakes, avoid contact for you and your pets.

Coastal and freshwater recreation monitoring factsheet | LAWA website
Ext. link

Avoid contact with potentially affected water

Recreational water users (paddling, kayaking, fishing) should be aware of potential blue-green algae (cyanobacteria) blooms when using local waterways

Algal blooms can develop rapidly. If the water looks discoloured, smells unusual, or if there is scum on the surface of lakes or near the shore, it’s best not to enter the water.

If you think cyanobacteria is present keep yourself and your pets safe by avoiding all contact with affected water.

Check LAWA Can I Swim Here, local health warnings, nearby signage and any visible signs of blue-green algae before coming into contact with the water (water discolouration, accumulation of scums).
Do not take or consume trout, or any fish, that are found dead.
Ensure that trout, or any fish, caught in the affected areas are thoroughly gutted and rinsed in clean tap water before eating.
Do not eat shellfish or kōura from affected areas.
It is advised to err on the side of caution and avoid water activities including riverside fishing and duck shooting until you can confirm the water is safe.

Public health alerts and warnings from Te Whatu Ora - Health New Zealand

You can also stay up to date with public health alerts and warnings issued by Te Whatu Ora - Health New Zealand, including cyanobacterial warnings for waterbodies.

Visit the website below, and use the filter to select public health alerts and your preferred location.

Check for public health alerts and healthcare news | Te Whatu Ora - Health New Zealand website

Get medical attention straight away if you are experiencing a reaction

Anyone who suspects they are experiencing a reaction due to contact with toxic algae should seek urgent medical attention and advise the doctor of the potential exposure to toxic algae.

In an emergency call 111.

If it's not an emergency but you have come into contact with affected water and have concerns about your health, please contact your GP or Healthline on 0800 611 116.

Concerns and enquiries

If you have any enquiries about your local drinking water supply, please contact your local district or city council.

Public health units work closely with local councils to monitor drinking water standards. If you are concerned about health issues associated with your water supply, please contact one of the below health units.

Bay of Plenty Public Health Unit
For queries regarding Lake Taupō or the upper Waikato River (from Taupō to Lake Maraetai) contact the Bay of Plenty Public Health Unit on 0800 221 555.

Waikato Public Health Unit
If your enquiry relates to a different location than above, call the Waikato Public Health Unit on 0800 800 977.