Swimmers and dog walkers are being warned of a toxic algal bloom in a river near Masterton, which could kill pets.

The bloom was discovered in the lower Waipoua River at a monitoring site at the Colombo Rd bridge on Wednesday last week.

"The bloom is at 15 to 20 per cent coverage of the riverbed, which is approaching our guideline alert levels", Greater Wellington Regional Council senior environmental scientist Mark Heath said.

"Small mats of it are lifting and floating along the bank. These mats are a concern because toxic algae can be deadly if swallowed. Dogs love the smell of them and will eat them if they get the chance. They need to be kept on a leash if you're down by the river."

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Toxic algae is potentially deadly to humans although there has never been a recorded case in New Zealand. But, as toxic algal blooms have increased in frequency and size in recent years, Heath is concerned babies and toddlers could be at risk if levels rise.

"When a river reaches red alert level, there's a lot of toxic algae washing up and we're starting to worry about small children picking it up and eating or mouthing it. It's very dangerous. There's also potential for kids horsing around in the river to accidentally swallow it."

Above Masterton, the Waipoua River runs through rural private land where Greater Wellington does not have monitoring sites. While the upper Waipoua is not as prone to toxic algal blooms as the urban stretch, people using the river there are advised to check the river and banks before swimming or dog walking, and to avoid contact with the algae.

Toxic algal blooms are caused by a combination of warm weather, high nitrogen concentrations and persistent low water flows. They are usually cleared or "flushed" by a period of high rainfall.

Greater Wellington will monitor all the region's rivers throughout summer.

Toxic algae warning signs will be placed at river entrances and there will be an awareness campaign helping people to identify it.

If or when rivers reach red alert levels, Greater Wellington will inform the public through social and traditional media.

Warnings were put out last year over much of the summer for the Hutt River, which had toxic algae in it.

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Drone footage of the river showed sweeping tracks of dark-coloured algae covering much of the riverbed in places.

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.

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

• Anyone concerned about health risks can consult their doctor or call the health line on 0800 611 116.