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A hot, dry summer naturally leads to lower-than-usual water flows and warm water temperatures – which increases the likelihood of toxic algal blooms in rivers, streams and lakes. This page explains what toxic algae is, what it looks like, how to reduce risk – and where to check the latest warnings and monitoring information.
The most common form of toxic algae found in Hawke’s Bay’s waterways come from cyanobacteria. Tiny organisms that occur naturally in many waterways, they’re a normal (and helpful) part of the ecosystem. But in hot, dry weather – especially when flows are low and water warms up – cyanobacteria can multiply rapidly.
Under these conditions, some types can produce toxins and form blooms. These may appear as discoloured water (more common in lakes and slow-moving water), or dark, slimy mats attached to rocks on riverbeds (more common in rivers).
We’re already seeing toxic algae in the 2025–26 summer – and with rivers running warm and sluggishly, further blooms are likely.
Toxic algae can harm people and animals, but children and dogs are most at risk. This is particularly the case in rivers, where young children and dogs are more likely to handle or put rocks in their mouths that may be covered in toxic algae. Mats can smell musty and enticing, and even a small lick can cause serious illness. Because toxicity can only be confirmed by laboratory testing – and conditions can change quickly – the safest approach is simple: treat any suspicious algae as toxic.
Living toxic algae: Thick dark brown or black mats attached to rocks under water |
Living toxic algae: Mats floating in shallow waters usually with air bubbles |
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Dead toxic algae: Attached or detached from rocks, the mats can be wet or dry and be grey, white, black, or brown
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“Potentially toxic algae” is a common term used to describe some types of bacteria known as cyanobacteria..
Cyanobacteria can photosynthesise and ‘bloom’ just like true algae. Some of these species can grow under the water on the riverbed, creating mats on rocks and other surfaces. Other species will ‘float’ within the water looking less like a solid mat and may cause the water to look green, blue-green, or reddish-brown, or form a surface scum.
Under certain conditions, toxic algae can produce toxins called ‘cyanotoxins’.
Contact with cyanotoxins through skin contact or by swallowing can lead to several symptoms from minor stomach cramps to serious illness. Young children are particularly at risk, as they are more likely to handle or put rocks in their mouths that may be covered in toxic algae.
Just like with humans, animals such as dogs and livestock are vulnerable to cyanotoxins if they come in contact with or ingest them. Animals can be exposed when they drink contaminated water, lick algae from rocks or shorelines, or swallow water while playing. Keep stock out of affected areas and provide alternative drinking water.
Unfortunately, mat-forming toxic algae often has a strong earthy/musty smell that is especially attractive to dogs. This tends to put free-roaming dogs at greater risk, as they can splash, sniff, and explore the water’s edge.
Our science team monitors water quality at 37 sites from Māhia to Pōrangahau from November to March. This monitoring focuses on faecal contamination, which helps assess whether sites are suitable for swimming. Some of these sites are also checked for potentially toxic algal mats, Microcoleus, where conditions are known to support their growth.
We publish results online and share them with LAWA which brings together swimming spot information and health warnings from across the motu.
Because toxic algae can bloom quickly – sometimes between sampling times – reports from the public are an important early warning system.
Confirming toxic algae requires sampling and laboratory testing, and toxin levels can change quickly. Not every site can be tested every day, and you can’t reliably tell toxicity just by looking. That’s why we recommend treating suspicious algae as toxic and avoiding contact.
If you think your dog may have been exposed, contact your vet immediately. Important: prevent your dog from licking their coat until they’ve been washed with fresh water.
It depends on contact. The risk increases if you are splashed, fall into, or swallow water.
If you do go out, minimise contact and wash gear down afterwards – but if a warning is in place, it’s best to avoid the water altogether.
No. Toxins may not be removed by boiling or common household treatment methods. Arrange an alternative water source.
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