Wairarapa river users are being warned to protect themselves and their pets from an outbreak of toxic algae, which is potentially lethal to dogs.
Weekly monitoring of the Waipoua River has revealed an increase in cyanobacteria, which can wash up on river banks in brown or black clumps.
Contact with the algae can cause vomiting, diarrhoea, skin irritations and other allergy-type symptoms in people, and it can be fatal for livestock or dogs if it is eaten.
Warning signs have been put up at key access points to the Waipoua River.
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Greater Wellington Regional Council aquatic ecosystems and quality team leader Juliet Milne said warm weather in Wairarapa and the lack of heavy rain in the Waipoua catchment were key factors behind the increase.
"The risk to river users and dogs is likely to remain until there's significant rainfall to flush the algae away."
People should avoid touching or swimming near the algae, and should contact their GP if they suspect a reaction to it.
Dog owners should put their pet on a lead and move away from the river's edge. If they suspect their dog has eaten the algae, they should take their pet to a vet immediately.