No one has been arrested or charged at this stage.
Detective Senior Sergeant Geoff Baber confirmed police were notified at 7.30pm on Sunday.
"A scene examination is being carried out at an address in Auckland's central business district," he said.
A baby boy is in critical condition in Starship Hospital with severe head injuries.
"Police are continuing to piece together what has led to this child's injuries which we believe at this stage are non-accidental.
"Police are speaking to several people in relation to this matter and there is no further information available at this stage."
Police would not comment on the background of the child's mother or whether she had been spoken to.
But the Herald understands the toddler is her sixth child - and her five older children have all been removed from her care.
Oranga Tamariki central Auckland regional manager Anna Palmer refused to answer any questions about the case, including when the agency first engaged with the injured baby's mother, when her older children were uplifted and why, whether the 17-month-old was being monitored and whether his case was under review.
"No one wants to see a child seriously injured, and our hearts go out to those who love him," said Palmer.
"We work to improve children's lives and prevent harm, and our people feel it deeply when a child suffers.
"We can't comment any further due to privacy reasons, and while a police investigation is underway."
Earlier this month the Children's Commissioner Judge Andrew Becroft announced a review into Oranga Tamariki's child uplift policies relating to care and protection issues for Māori babies.
It follows controversy over the attempted uplift last month of a young Māori mother's baby from Hawke's Bay Hospital that today saw Minister for Children Tracey Martin announce an internal inquiry.
The "thematic review" by will look specifically at policies around Māori infants aged 0-3 months.
Judge Becroft said while the review would initially focus on the 0-3 months age group, he could not rule out extending the review to older children.
He said his office had a statutory mandate to investigate.
"If we didn't do it we would be asleep at the wheel," he said.
Earlier the same day Minister for Children Tracey Martin announced an internal inquiry by Oranga Tamariki into its processes specifically around the Hasting family's case.
Martin could not comment on the 17-month-old's case while it was under investigation.
