Deadly toxic algae has reached extreme levels in Kāpiti and the Wairarapa, covering more than 50 per cent of the Waingawa and Ōtaki riverbeds.
It is the first time guideline levels have been exceeded in Ōtaki River and the affected area extends from Hautere all the way to the river mouth.
The stretch of the river below State Highway 1 is particularly bad and swimmers should avoid the area, the Greater Wellington Regional Council warned.
It takes only a coin-sized piece to kill a dog.
Detached mats of dried algae are widespread along the river margins and have also been found in the Waikanae River at Jim Cooke Park.
Swimmers should also avoid the Waingawa River, in Wairarapa, and be cautious when choosing a swimming spot in the Waikanae River downstream of SH1, due to the presence of detached mats.
The Greater Regional Council is warning people to not swim and keep their dogs away from the rivers.
Dogs should be kept on leashes by the Pakuratahi River from SH2 to its junction with the Hutt River in Kaitoke Regional Park. People were advised not to swim in the area.
The Waipapa Stream [near rocks]and the Hape Stream also on the Red Rocks walking track [near the start of the track] both had significant blooms. The streams were not swimmable and dog walkers should be careful.
Cyanobacteria – or toxic algae – is more toxic than cobra venom and it takes only a coin-sized piece to kill a 20 kilogram dog.
Last year, the council's freshwater scientist Mark Heath said about 150 dogs had died "an agonising death" in the past 15 years from eating the algae.
Although unlikely, he feared what would happen to a child who got some on their hands or ate a piece.
The algae was shiny brown/dark green and coated submerged river stones. When it died it floated to the surface and formed brown mats at the water's edge.
On Thursday the council warned it was important to keep an eye on babies and toddlers who were inclined to put objects in their mouths, and seek emergency medical attention if swallowed.
Dogs were at greatest risk because they loved the small of the algae, they should be kept on leads in the affected areas and taken to the vet immediately if they eat it.
SYMPTOMS
Contact with toxic algae can cause skin irritation, nausea, vomiting, numbness, tingling or muscle twitches.
Affected people may feel short of breath. Swallowing toxic algae has the potential to cause serious health issues such as convulsions or loss of consciousness: symptoms which need urgent medical attention.
If you develop these symptoms following contact with the river, please contact your doctor or Healthline on 0800 611 116.