A red alert for toxic algae is in place for Te Awa Kairangi/Hutt River after large algal blooms and detached algal mats were found from Poets Park to Silverstream Bridge.

An amber alert has also been issued for the Wairarapa’s Ruamahanga River at Double Bridges.

The Kāpiti Coast’s Waikanae River at Jim Cooke Park remains on red alert, while an amber alert is still active for Ōtaki River at State Highway 1.

Greater Wellington environmental science Senior Advisor Penny Fairbrother said the public should check for toxic algae before entering these rivers.

“With the warm weather, toxic algae is spreading across our region. It can kill dogs and make people sick. Swimmers and dogs should stay out of the water where alerts are issued until further notice,” Ms Fairbrother said.

Toxic algal mats grow on rocks in the riverbed and form velvety dark green or black mats, which can break off and accumulate on the riverbank.

As detached algal mats dry out, they become light brown and emit a musty odour that attracts dogs.

“Dog owners walking near these sites should keep their animals leashed. Do not let your dog scavenge on the riverbank. Eating an algal mat the size of a 50-cent coin can kill your dog,” Ms Fairbrother said.

Greater Wellington is currently monitoring popular swimming spots on a weekly basis to make sure the community knows when and where it’s safe to swim.

As well as erecting warning signs, council officers have installed ‘Toxic Algae Risk’ signs at the highest-risk river locations, which can be adjusted to display the alert level.

To know if an area is suitable for swimming visit the LAWA website and check for warnings and alerts in the Wellington region.

To learn how to identify toxic algae visit Greater Wellington’s toxic algae webpage.

If you have been in contact with toxic algae and are feeling unwell see your doctor or ring Healthline on 0800 611 116.

If you think your dog has swallowed toxic algae, especially if it is showing symptoms such as breathing difficulties, take it immediately to the nearest vet.

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