Toxic algae blooms found in Waianakarua River and Butchers Dam

Swimmers and dog walkers are urged to be wary of toxic algae in Otago lakes and rivers.

Otago Regional Council has confirmed toxic algae blooms in Waianakarua River and Butchers Dam.

Its lakes scientist, Hugo Borges, said these kinds of algae could make humans and pets sick, and they could be fatal for dogs.

"Toxic algae, or cyanobacteria, thrive in warm, nutrient-rich conditions and when river flows are lower than normal, so these blooms are fairly common throughout all of New Zealand at this time of year," Borges said.

"In lakes and ponds it looks like pea soup, and in rivers it looks like black-brown tar on rocks and in mats on the riverside."

He urged people to stay out if in doubt.

"Our best advice is for people to learn how to recognise these algae and practice caution around lakes and rivers in summer.

"If ingested, toxic algae can cause significant illness and can be fatal for dogs, who can be particularly attracted to the smell of algal mats from rivers."

While two locations had confirmed toxic algae blooms, Borges said they could appear anywhere in Otago.

Algae was present in smaller amounts for most of the year, but they bloomed and caused issues in the right conditions, he said.

"This can happen anywhere in the region, and while council staff look out for algae during the weekly water quality checks in summer, we can't cover every waterway in Otago all of the time.

"That's why it's important for people to learn how to identify toxic algae, stay vigilant, and report sightings of potentially toxic algae to council."

Signs that a dog may have consumed toxic algae include lethargy, muscle tremors, fast breathing, twitching, paralysis, and convulsion.

They should be treated as an emergency and referred to a vet immediately.

Anyone experiencing a reaction should seek urgent medical attention.