Greater Wellington senior environmental scientist Dr Mark Heath said toxic algae had increased to dangerous red alert levels in the Waipoua River over the past week, with detached mats washing up at the river's edge.
"We strongly advise against swimming and letting your dog off the leash. It is very likely as the weather gets warmer that this risk will increase.
"It's important we all scrub up on the facts so we can all keep safe this summer, as toxic algae can be harmful to people and dogs.
"Algal mats grow on the rocks in the riverbed and form leathery dark green or black mats, which can break off and accumulate at river edges.
"As the algal mats dry out they can become light brown colour, and have a distinctive deep earthy or musty smell."
Dogs were mostly at risk because they liked the smell and taste of toxic algae. Even a small amount - about the size of a 50 cent piece - can be enough to kill a dog.
Owners especially needed to be vigilant of their dogs sniffing out toxic algal mats which can wash up at river edges.
Toxic algae was not the only harmful thing lurking in the water this summer, as bacteria from leaking pipes or run-off after heavy rain can contaminate swimming areas.
"In general, a good rule of thumb is to stay out of fresh or seawater for 48 hours after rain, and always check for warning signs," Heath said.
Greater Wellington monitored popular swimming spots on a fortnightly basis to make sure the community knew when it's safe to swim.
If you have been in contact with toxic algae and are feeling unwell see you doctor or ring Healthline 0800 611 116. Seek urgent medical attention for anyone with breathing difficulties or convulsions. If you think your dog has swallowed toxic algae, take it to the nearest vet immediately.
