A toxic algal bloom in the Waipoua River poses warnings for any swimmers in the greater Wellington region.
The Regional Council says the risk is not extreme but people shouldn't swim in the water or let their dog off its leash.
"We've detected the toxic algae above guideline levels in the Waipoua River. Fortunately, there are no detached mats washing up at the river's edge so at this stage the risk is not extreme," senior environmental scientist Dr Mark Heath said.
Dogs are mostly at risk because they are attracted to the smell of the algae, and even a small amount can be enough to kill them.
"Algal mats grow on the rocks in the riverbed and form leathery dark green or black mats.
"These mats can become unstable and slough off the rocks and accumulate at river edges. As they dry out they can become a light brown colour, and are distinctive for their deep earthy or musty smell," Dr Heath said.
"Owners especially need to be vigilant of their dogs sniffing out toxic algal mats which can wash up at river edges."
He said the council monitors swimming spots for algae, E coli and enterococci regularly to keep the community informed of any risk.
"In general, a good rule of thumb is to stay out of fresh or sea water for 48 hours after rain, and always check for warning signs," Dr Heath said.
The bloom may get worse as the weather gets warmer.
The council urges anyone who thinks they may have swallowed the algae to seek immediate medical attention.