Inspector Shawn Rutene said the boy had run out into the road and was hit.
"What's occurred here is that a young child, age of 3, has run across the road in front of a moving vehicle and as a result of that, that child is in Starship Hospital with serious injuries."
It was not a driveway incident, he said.
Jaemont Ave was closed for several hours yesterday while the Waitemata Serious Crash Unit investigated the scene.
St John ambulance also attended and road workers in the area stopped their work while police examined the area and made inquiries.
Both the driver and the family of the boy were being supported by police.
Rutene wanted to remind people to be extra careful on the roads at the moment.
"We're heading into the school holidays.
"Users of the road, parents and caregivers need to be vigilant in and around these areas - even these quiet streets."
A resident who lives about 20m from where the accident occurred, Peter Baran, said he was told by the police that a small boy had run out and been hit by a vehicle, he had been taken to hospital and thought his injuries were serious.
He did not know who the boy was.
Baran said police were examining what looked like a station wagon.
Another resident of Jaemont Ave, Terrance Caples, said there had been a serious accident and the road had been blocked by police.
"There didn't appear to be any damaged vehicles and no glass (on the street)," he said.
