"There was a car in the middle of the road, going down the hill. I was going up and the house was right on the hill.

"The pram went flying down the hill right in front of me. I slammed on the brakes but it was too late," he said.

"I didn't see it coming ... I slammed on the brakes but it was too late."

The infant was flown to Starship children's hospital with critical injuries after the crash but died in hospital this afternoon, police say.

The basket in the stroller came free in the collision and was flung around three car lengths forward, a witness said.

Saksena said he was driving between 20km/h and 30 km/h along Lake Rd in Northcote before the collision.

"I pulled over and put the hazard lights on and went to see the baby," he said.

"There was no one there, and then the mum rushed out to see what had happened.

The child's mother told Saksena she'd forgotten to put the brakes on the stroller, he said.

A staff member from a nearby childcare centre rushed out to the crash site and began to perform CPR on the baby, Saksena said.

"The mother was in shock ... then she asked what happened and she started to cry."

Emergency services were called to the North Shore street at 8.54am.

Kevin Walker said he was driving on Lake Road with his wife when the collision occurred.

"You just see a pram came out the driveway. In your head you're like 'Am I dreaming?' It was kinda like a nightmare.

"I saw the mum come running, she was in a panic, everybody was."

He said the fast-moving pram ran through a narrow gap between two moving cars in his lane but was then struck by a vehicle coming the other way.

"You could probably have that happen 100 times and it would not have hit anything.

He said the vehicle which struck the pram was not driving fast.

"He tried his best to brake. But it hit the middle of the car ... and the basket went flying, about 50 feet."

He added: "Everything just stops when you see something like that. It's just absolutely heartbreaking."

Jackie, a resident who lived nearby, came past the scene in the evening to leave some flowers at the side of the road.

She did not know the family well but said such a tragic death affected a whole community.

"Five months old - it's just devastating."

Jackie said she felt sorry for the driver of the car.

"There's just nothing they would have been able to do."

A witness to the crash earlier told the Herald the stroller had to be pulled from the van it hit, and there was a large dent in the front of the vehicle.

Ali, who didn't want his last name used, said other motorists rushed to try to help but there wasn't much they could do.

Saksena spoke to police after the incident and tried to continue his working day in an effort to distract himself.

"I didn't know what to do. I went to work but I couldn't do anything, so I just went home."

"I can't sleep, I can't drive, I can't do anything."

The child's death will be referred to the Coroner and police are ensuring those involved are being provided with support.