Hutt Valley residents are being warned to stay out of the river due to a rise in potentially fatal toxic algae.

Video from the Greater Wellington Regional Council shows vast patches of the dark algae spreading across the Hutt River.

Greater Wellington and Regional Public Health say swimmers should keep out of the river below Kaitoke to avoid ingesting any of it.

"Hot weather, warm water and a lack of rain in the last two weeks have made conditions ideal for the native algae to suddenly bloom," said Greater Wellington environmental scientist Dr Mark Heath.

"This means pieces of algae are breaking off rocks and floating downriver in much larger quantities than usual, and this has become a risk to swimmers as well as dogs.

The dark shadow in the water is toxic algae, which, in extreme cases, could be fatal to humans if swallowed. (Photo/Greater Wellington Regional Council)

"Even very small amounts of toxic algae, if swallowed, can cause serious illness and - although unlikely - can potentially be fatal. Usually algal mats are in limited quantities and easily avoided by swimmers, so there is a low risk of actually swallowing one.

"This week we have seen a sudden increase in the number of mats floating down the river and, without rain, this is likely to get worse. So the chance of accidentally swallowing an algal mat is much higher than usual, and we think this justifies a warning to stay out of the river."

The council is also encouraging dog owners to keep their dogs on a lead and away from the river's edge.

"The algae gives off an earthy, musty smell that attracts dogs. It usually gets caught up in rocks on the river bank so, with the volume of algae we are seeing, we encourage owners to keep their dogs safe on a lead and well away from the water."

At this stage the warning is only for the Hutt River below Kaitoke. The council is monitoring other rivers in the region, and any warnings will be put on the Greater Wellington website.

"A burst of significant rainfall should flush out the river and lower the risk," Heath said.

The most likely symptoms of algal poisoning are nausea, vomiting, numbness, tingling, muscle twitches, shaking, weakness, breathing difficulties and, although unlikely, potentially convulsions and loss of consciousness.

People should stay out of the river and keep their pets away as well, until rain flushes out the algae. (Photo/Greater Wellington Regional Council)

These symptoms would usually occur soon after exposure, according to Dr Jill McKenzie of Regional Public Health.

"If you are concerned about potential health risks, consult your doctor or ring the health line - 0800 611 116. Your doctor should report potential illness from toxic algal exposure to Regional Public Health," she said.

Upper Hutt Mayor Wayne Guppy said he hadn't seen the algae bloom get so severe this early into summer before.

"I mean, the weather has been outstanding, but it's, again, disappointing that so early on in late spring, early summer that we're faced with quite a substantial infiltration of the algae through the Hutt River."

Guppy said it was an "ongoing issue".

"It's early on, it's not ideal, but we all need to take those precautions about getting into the river. What we need to fix it, like it always is, is what you don't want, is rain and a good flush out."

Guppy said there were a "multitude" of reasons the algae bloom got so bad, but part of it was down to humans.

"It's one of the side effects of, you know, some of the things that we've done to the world, and we have to live with it in the Hutt Valley."

It was important for people to take note of the warnings, and keep their pets out of the water as well.

"Just a couple of days ago you're wandering down the river and you're watching people through sticks in for their dogs. I know everyone wants to do it but it's more tragic when someone loses the family pet, so just take heed.

"The rain will come, it will get flushed out, and we'll be able to get down there and swim and enjoy the river."