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, Stuff reports.
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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.