Those gearing up for a summer of cooling off in their favourite local swimming hole are being warned to check first for signs of blue green algae.

Photo: RNZ / Conan Young

The toxic cyanobacteria Phormidium has been coating riverbeds and lakes in a thick black mat for the past decade.

It has a similar level of toxicity to cobra venom - just a teaspoon of it can kill a 25 kg dog - and so far about 100 family pets had fallen victim to it in New Zealand.

No part of the country had been spared with 103 rivers closed nationally because of the algae in the past six years.

So far the early summer the country had been experiencing was providing the warm water the alga needed to thrive.

Already authorities had been forced to close popular swimming spots from Lake Taupo - where the swim leg of a Half-Iron Man event had to be cancelled - to South Canterbury's Opihi River, closed this week after a long dry spell in the region.

Material provided by the Canterbury Regional Council painted a picture of cyanobacteria occurring "naturally" and talked about it building up due to long periods without floods that were needed to wash it away.

But University of Canterbury doctoral student Tara McAllister, who had been trying to get a better understanding of what caused the alga, had so far worked out there were a number of factors at play.

Photo: RNZ / Conan Young

"Typically a lot of our rivers would have had ... trees planted all along the side of rivers but now a lot of those are removed so there's more sunlight, the temperatures are higher, [there is] more sediment in the river, associated land use, things like farming, we end up with a lot of sediment and nutrient input in to our rivers ... in Canterbury the big thing is irrigation."

The phosphorus found in sediment, often generated by farming and other forms of development, was an important factor she said.

"It [Phormidium] uses the phosphorus attached to sediment to actually grow, so it doesn't use the phosphorus from the water, it actually traps sediment and uses the phosphorus that is stuck on to the sediment to grow."

No one thing was to blame and the impact of each factor often depended on the river, but one thing it was not, was a natural process, she said.

"When it's just around in small abundances, you know growing on this rock, growing on that rock, that's what I would consider natural, but when you go to a river and the whole river's black, then I would consider that quite unnatural."

Photo: RNZ / Supplied

One thing that was clear was the rate at which it spread with the algae growing from a teaspoon full to a thick mat covering a large rock within just seven days.

Tara McAllister said this was a challenge for authorities charged with monitoring waterways because they wouldn't always be able to pick up on its presence in time.

"I really would encourage everyone to, even if there's no warning at a particular site, if you're going to a river and you have toddlers or a dog, you really need to check it out for yourself before even getting in there."

Symptoms included skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.

In extreme cases dogs could die within 30 minutes and anybody finding sick canines by rivers was urged to take them to a vet immediately.