Algal bloom

During the warmer months lakes, rivers and coastal bays can be affected by blooms of algae. Most of these blooms are harmless even if we can see them, but some can be toxic.

Blooms may develop rapidly. When they decompose they may cause a bad smell and affect other lifeforms in the water. Some blooms can be harmful to humans and animals, such as dogs, especially when they break down.

It is best to avoid contact with the water if:

  • it looks discoloured
  • has an unusual smell
  • has green or brown particles suspended in it, or
  • if you can see unusual scum or leathery mats of algae on the surface or on the bottom of the waterway

If you feel unwell as a result of contact with algal bloom, you should contact your doctor, or call Healthline on 0800 611 116.


Reporting algal bloom

If you are concerned about an algal bloom:

  • In Northland - Te Tai Tokerau call Northland Regional Council on 0800 504 639.
  • In Auckland - Tāmaki Makaurau call Auckland Council on 09 301 0101.


For more information

Auckland Council - algal bloom alerts

Northland Regional Council - algal bloom alerts and sampling

MPI - Shellfish biotoxin alerts

While it may look unpleasant and sometimes smell, most algae in our waterways is harmless. However, some species found in our waterways and oceans can be harmful to people and animals if ingested.

Blooms are caused by sudden, massive growths of naturally occurring, microscopic plant and animal life (algae and plankton). They can result in the discoloration of fresh and salt water bodies. They occur extensively over summer periods and come in many different colours.

Blue-green algae (or cyanobacteria)

Blue-green algae (or cyanobacteria) are naturally occurring microscopic organisms that live in a range of aquatic environments, from near-pristine to those more impacted by land use. They occur naturally in New Zealand waters. An increasing number of cyanobacterial species are known to include toxin-producing strains. These natural toxins, known as cyanotoxins, are a threat to humans and animals when consumed in drinking water or coming into contact with the skin during recreational activities.

It is very difficult to predict or explain where blue-green algae may occur, as well as their size and duration. This is because numerous environmental conditions need to be met to enable the rapid growth of the cyanobacteria (calm weather conditions, plenty of light, warm seawater temperatures and sufficient nutrients to sustain their growth), followed by the right conditions to dislodge blooms (ie stormy weather). Once dislodged, factors such as the tidal conditions and wind direction need to be understood to predict where blooms may eventually be deposited. These requirements mean blooms most commonly occur during spring and summer, and are a natural phenomenon in shallow, productive coasts. 

Marine algal blooms

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 be harmful when people or animals come into physical contact or ingest seafood which have been exposed to these toxins.

In some circumstances certain blooming 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.

Toxic algae can be harmful to your health if it is swallowed or if it comes into contact with your skin and body. It can cause:

  • fever
  • headaches
  • dizziness
  • skin rashes
  • eye irratations
  • stomach cramps
  • aggrevated hayfever and asthma
  • breathing (respiratory) irritations
  • damage to the nervous system
  • gastroenteritis symptoms (diarrhoea, vomiting or abdominal pain)

Dogs are also particularly at risk of serious illness if they eat or have contact with toxic algae. They should be kept away from the water if it looks like there may be a bloom. They may also eat algal mats that accumulate in and around rivers.

Shellfish can also become toxic if they have been in contact with a bloom. Shellfish are considered a high-risk food because they can harbour bacteria, viruses, biotoxins and chemicals, that may be present in the water. Cooking shellfish thoroughly will kill bacteria. However, it won’t destroy biotoxins and might not eliminate other contaminants that could be present.

If you feel unwell as a result of contact with the bloom, you should contact your doctor, or call Healthline on 0800 611 116.

If you get sick after eating shellfish, make sure you:

  • phone Healthline for advice on 0800 61 11 16, or get medical help immediately
  • contact us
  • keep any leftover shellfish in case it can be tested.

If you come across algal bloom:

  • Avoid contact with the algae, including wading in affected areas, and ensure children do not play in it

  • If you feel unwell as a result of contact with the bloom, you should contact your doctor, or call Healthline on 0800 611 116 

  • Do not gather or take algae from the beach for things like using on your garden

  • Keep dogs away from the algae and any decomposing material which could cause a reaction

  • Do not gather or consume shellfish from the affected areas

In Auckland - Tāmaki Makaurau, Auckland Council issues warnings when toxic algal bloom is detected at high levels in Auckland's lakes, rivers and beaches.

In Northland - Te Tai Tokerau, Northland Regional Council conducts regular sampling of Cyanobacteria (blue-green algae) at 15 State of the Environment lakes as well as popular swimming spots and issues alerts where necessary. It also responds to public reports of algal and plankton blooms in marine and freshwater.

MPI issues warnings when there is a risk of serious poisoning from shellfish toxins and for other kaimoana. 

You can also check Safeswim for up to the minute information on water quality and swimming conditions at your favourite swimming spots in Auckland and Northland.

If high concentrations of algal bloom are found a health warning will be issued by the relevant council and National Public Health Service - Northern Region advising people:

  • not to drink or use the affected water
  • to avoid contact with it
  • to keep pets (e.g. dogs) and livestock out of areas of lakes or rivers affected by blooms
  • if necessary, to avoid collecting shellfish and other kaimoana from contaminated sea and river beds

If you feel unwell as a result of contact with algal bloom, you should contact your doctor, or call Healthline on 0800 611 116.

The Ministry for Primary Industries (MPI) test shellfish and seawater for toxic algae every week from popular shellfish gathering areas around New Zealand. If the shellfish are not safe to eat, MPI issue public health warnings and put up signs at affected beaches.

Last updated 26.3.2024

For health advice call Healthline for free anytime on 0800 611 116
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