Swimmers and dog owners are on high alert with toxic algae on the rise in Wellington rivers.

The algae, which are deadly for dogs, are "quickly intensifying" throughout the region, with all monitored rivers reaching health alert levels, Greater Wellington Regional Council said.

Greater Wellington's Otaki, Waikanae, Hutt, Ruamahanga and Waingawa river monitoring sites have exceeded the 20 per cent alert threshold, and the Waipoua River is above the 50 per cent "no swimming" line.

Detached algae mats have been seen in all the rivers.

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"Our message is clear. People should stay out of the Waipoua River and remain vigilant in other rivers," said Greater Wellington senior environmental scientist Dr Mark Heath.

"Levels are expected to increase, with hot dry conditions forecast for much of the region throughout next week.

"This comes on top of a prolonged dry spell. With the exception of the Ōtaki River, it's been 35 days since the last decent flush for most rivers and streams in the region, which has produced ideal conditions for the growth of toxic algae."

Warnings are being placed on the rivers shown below.

• Otaki River, warning extends from State Highway 1 bridge to river mouth

• Waikanae River, warning extends from Main Rd (old SH1) to river mouth

• Hutt River, warnings for Melling and Silverstream sites

• Waingawa River, warning for south road site

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• Ruamahanga River, warning for Morrisons bush site

• Waipoua River, warning extends from Paierau Rd to confluence with the Ruamahanga

Toxic algae grow on submerged river stones, presenting with a shiny brown or dark green to violet coating. They can also become unstable and detach, floating to the surface forming small brown or black mats at the water's edge.

Given conditions quickly change, people should be remain vigilant around rivers and streams throughout the region.

"This is the prime season for toxic algae, so look out for algae covering rocks and for detached mats in the water and lining riverbanks. If in doubt, stay out of the water and keep a close eye on children and dogs," Greater Wellington said in a statement.

"It is important to keep an eye on babies and toddlers who may put objects in their mouths. Seek emergency medical attention immediately if anyone in your group swallows toxic algae.

"See your doctor or contact Healthline 0800 611116 if you have been in contact with toxic algae and develop the following symptoms: nausea, vomiting, numbness, tingling, muscle twitches, shaking, weakness. Get urgent medical attention for anyone with breathing difficulties, convulsions or loss of consciousness."

Before you swim, stay safe by finding out about toxic algae at http://www.gw.govt.nz/safeswim/.

For more information on where it is safe to swim, go to: https://www.lawa.org.nz/explore-data/swimming/