People are encouraged to educate themselves on the signs of cyanobacteria poisoning amid concerns climate change may see cases rise.
Photo: File photo / 123rf
Found in waterways, the algae is one of the most toxic natural compounds in the world.
About 200 dogs have died from toxic algae in New Zealand the past decade, but the human impact is not as widely known.
Just a teaspoon full of cyanobacteria was enough to kill a dog, Environment Canterbury water quality team leader Shirley Hayward explained.
"The challenge with dogs is that many of them love the water but some of these cyanobacteria that produce toxins also produce odours that dogs are very attracted to.
"So they are actually drawn to some of these scums and they like to have a lick and a taste. That's why they're particularly vulnerable."
If a dog is poisoned, disorientation or unusual behaviour, vomiting, diarrhoea or seizures are tell-tale signs.
Veterinarian Association veterinary services head Sally Cor said it was important owners acted quickly.
"The best thing to do is to contact the vets as soon as possible if your dog is symptomatic or if you are worried. Just get them in the car, start driving to the closest vets clinic and just have someone call ahead so that they can be prepared," she said.
Cyanobacteria appears as sticky, black or dark brown mats on river beds, or as an olive-green scum across lakes and ponds.
The river variety tends to be more toxic, but both can cause serious illness in pets and people.
Dr Susie Wood is a freshwater scientist at the Cawthorn Institute, and an expert in cyanobacteria.
Eating the algae caused the worst symptoms - neurological impairment and organ failure - but touching or swimming in it could have an effect too, she said.
"You can experience some skin irritations, so people will have rashes and in some instances, respiratory problems.
"There are two different types of toxins - ones which impact you when you touch them or have a lot of contact with them. And the other, which is more likely to be fatal, happens when we ingest them."
For this reason, young children playing near water were most at risk, especially as they were more likely to accidentally ingest some of the algae, Wood said.
She said there was also a lack of awareness around cyanobacteria effects on people, so cases tend to go unreported.
"Often some of the symptoms if you just had a mild exposure to the toxin or the algae, are gastroenteritis, stomach problems, vomiting and diarrhoea.
"The first thing you think about is not necessarily algae - it might be what you had for lunch or whether you were exposed to another bacteria on the way to the lake or river."
Cyanobacteria is regulated in drinking water standards and most councils routinely test for it at popular parks and swimming spots - notifying people through signage or online updates.
The Land Air Water Aotearoa (LAWA) site covers hundreds of locations across the country.
Blooms were more common in summer but it was highly unpredictable, and climate change may make it a year-round issue, Hayward said.
"The situation can change very quickly and that makes it very difficult for monitoring in that one day it might look fine and then next a bloom may have formed," she said.
"So it can happen that quickly under the right conditions. And it can happen anywhere, in any waterway."
Hayward implored people to always check waterways before they or their dog went swimming or had a drink. The golden rule: if in doubt, keep out.