A 5-month-old baby has died after being hit by a van in Auckland this morning.

The child was critically injured and treated by St John paramedics at the scene on Lake Rd, Northcote, before being taken to Starship children's hospital.

The baby later died of its injuries in hospital, police say.

Ali, who asked not to have his last name used, said a stroller raced down a driveway and over the road and into the other lane before being hit by a van.

The basket in the stroller came free in the collision, flying around three car lengths forward.

"The pram came down like a rocket," he said. "All of a sudden you see the baby flying through the air.

"It was a beautiful morning, then something like that happened. For a second I was thinking, 'what the hell, was that for real?'

"It was something you only see at the movies. It wasn't good; it's shocking."

The collision took place on Lake Rd, south of the Exmouth Rd roundabout.

The Police Serious Crash Unit had been at the scene investigating, a police spokesperson earlier confirming a young child had been hit.

Ali said the van that hit the child was not travelling quickly, but figured the momentum from the stroller coming down the steep driveway propelled the baby forward.

He said the stroller had to be pulled out of the van it collided into, with a large dent left in the front of the van.

Other motorists were quick to try and help but there wasn't much they could do, Ali said.

The baby must have had a belt on, Ali said, because they didn't fall out of the basket as it flew through the air.

He was not sure how the stroller ended up rolling down the driveway.