The slimy toxic algae benthic cyanobacteria on a rock in the Hurunui River.

It's not just the heat that's been worrying Dr Mark Heath these past summer nights.

The black, velvety mats of algae on the sides of the Hutt River, near Wellington, are what's preying on the scientist's mind.

More toxic than cobra venom, it takes only a coin-sized piece to kill a 20kg dog, Heath said.

ROBERT KITCHIN/STUFF The Hutt River taken from drone in Silverstream. There have been two toxic algae alerts this summer.

"And if we talk about a 20kg dog we could just as easily be talking about a 20kg child."

READ MORE:

* Health warning for toxic algae in Ashley River

* Taupō Ironman swim leg scrapped due to bloom

* Hutt River deemed safe as algae clears

Cyanobacteria – or toxic algae – has been the scourge of the capital's summer with the Greater Wellington Regional Council twice urging swimmers and their pets to stay away from the river.

SUPPLIED Mark Heath says the public need to be able to recognise toxic algae so they can avoid it.

Heath, the council's freshwater scientist, is disappointed that despite the warnings, at least two dogs have already died after eating the algae. Nationwide, about 150 dogs have been killed in the past 15 years, he said.

It's an agonising death and, although he said it was unlikely, he feared what would happen to a child who "got some on their hands" or ate some of the algae.

"This scares me just about every night when we have big algal blooms."

JOHN BISSET/STUFF Toxic algae signs are one of the ways the regional council alerts swimmers.

For now, scientists don't know exactly what the algae could do to humans, but Heath said one thing was certain, should a person swallow a piece of the floating bloom.

"There's a really big risk that someone could become very ill or sick."

The algae was a neuro-toxin and killed by blocking nerves from communicating to each other. In extreme cases, dogs die within 30 minutes of eating the algae.

HANNAH WILTON/SUPPLIED Kate the dog enjoys a swim at the beach after toxic algae alerts warned the rivers may not be safe.

What is toxic algae?

Toxic algae occurs everywhere but it's during hot weather and low water that the problems start.

When it comes loose from the river bed and brown or black mats wash up on the banks or form floating rafts is when it becomes dangerous, Heath said.

NICHOLAS BOYACK/STUFF Caroline Angus and Lucy at the Hutt River near Melling. Angus was surprised to hear that there was a toxic algae alert for the Hutt River

"As the mats start to die the cells break open and release these odour compounds which actually seem to attract dogs for some unknown reason."

The council tested the district's water weekly but people needed to recognise what the algae looked like so they could keep themselves safe.

"Sometimes you can smell it when you open your car door down at the river."

NICHOLAS BOYACK/STUFF Kasha Isaia, 11, jumps into the Hutt River. The popular Taita swimming spot, known as Taita Rock, has been quieter than normal with the river closed due to toxic algae. At noon on Monday, about 30 local kids were swimming and jumping off the rock.

He said the council did everything it could to alert people to the algae's presence with the use of traditional and social media, interactive maps, signs and website information, but the public had to do their part too.

"Do your research so you can identify it."

Deadly to dogs

Dogs die from seizures, severe vomiting and diarrhoea after they eat toxic algae, Pet Doctors New Zealand clinical director Dr Callum Irvine said.

"People perceive that the dog actually has to get into the water but it isn't the case, they will eat it from the banks."

Irvine said many owners didn't know their dog had ingested it until it was too late.

There is no vaccine or antidote but there's still an easy way to stop your dog from eating it.

"The bottom line is quite simply to stay away. If you know it's there then don't take the risk."

Why the Hutt?

Put simply, the river offers just what algae likes: a large gravel bed, low concentrations of phosphorus and moderate concentrations of nitrogen.

With no rain to flush the river clean and the hot sun drying out the blooms, this summer has been the perfect storm for the Hutt River, though Heath said it wasn't alone.

"The Hutt is very much in the public spotlight – it goes past two fairly big cities – where other rivers don't have that level or recreational use."

Somewhat ironically, the algae grows in good quality water and, with an estimated one million annual users of the river, it was imperative people heeded the warnings.

"We'd love to put a warning sign up on every post but we can't."