Auckland Council's pollution team is investigating the water at a west Auckland stream after reports a boy and his dog got sick less than a day after visiting.
However, the council says it's too soon to say if the water caused the illness.
The child's mother, Tracey Finlay, in a social media post said the boy was left vomiting and their labrador had seizures within 12 hours of playing in the water at the Kakamatua Inlet, Fairfax reports.
Both were recovering on Thursday.
A spokesman for Auckland Council said its pollution response team had been notified.
"While prior to this we have not been made aware of any issues at this particular location, as a general rule we advise against swimming in coastal streams," he said.
"They have a tendency to suffer from pollution issues and are not considered safe for swimming in."
No possum control programmes were currently taking place in the area, he said.