Dennis and Karen Casey with border collie Rex, beside Masterton's Waipoua River, found to have toxic algae fatal to dogs.

A Wairarapa river found to have toxic algae fatal to dogs and dangerous for humans should have more warning signs, according to tourists walking their pet beside it.

The Greater Wellington Regional Council warned on its website and social media pages on Thursday morning that toxic algae has been found growing in the Waipoua River near Colombo Rd in Masterton.

If eaten it can kill livestock and dogs, while touching it can cause vomiting, diarrhoea and skin irritations in humans, the alert said.

CALEB HARRIS/FAIRFAX NZ Masterton's Waipoua River under the Colombo Rd bridge, where, the public was warned, toxic algae has been found.

It said Masterton District Council had posted warning signs at key access points along the river, but on Thursday afternoon tourists and others walking their dogs near the Colombo Rd bridge over the Waipoua had no idea of the danger.

"Where are the signs? I wouldn't be impressed if I ended up with a big fat vet bill, and it wouldn't be a nice way for him to die," said Christchurch woman Karen Casey of her border collie, Rex.

There were no warning signs in the grassy parking area beside the river's stop bank where she and husband Dennis were exercising Rex while visiting family for Christmas.

But a sign on the other side of the bridge near a path to the river, did spell out the danger, and was backed up by a slightly putrid smell from the water.

The regional council alert said there were "moderate" levels of brown or black toxic algal mats growing on the river bed, after rising temperatures provided ideal conditions for its growth.

People should keep dogs on a lead and take care near the water.

The algae was found during monitoring carried out by the council along with Regional Public Health and local councils, done weekly in waterways around the Wellington region during summer.

People could check the cleanliness of waterways on the "Summer Check" section of the regional council's website.