'The dad says it's his fault': Witnesses recall horrific moment little boy killed by 4WD

The heartbroken father of a four-year boy killed by an out-of-control four wheel drive in a Melbourne car park has blamed himself, according to a witness.

Just moments after leaving the Epworth Health Care Centre with his parents, the boy, holding his dad’s hand, was left fighting for his life after being struck by the Landcruiser.

Motorist Cameron Smith and his wife Sarah were driving behind the land cruiser and watched it all happen.

"It's gone over the median strip and it's gone airborne about six or seven metres," he said.

"I've run over to assist the driver not knowing there was a little boy laying there."

Mother-of-two Sarah was inconsolable at the scene.

"I remember the dad saying it's my fault, it's not his fault," Sarah told 7 News as she broke down in tears.

Police reiterated her message, insisting the freak accident happened in just the blink of an eye.

"The father has gone out with his son, holding hands, doing what good dads do and this outcome has come up on them very, very suddenly," Detective Sergeant Mark Amos said.

Two adults were also seriously injured after being hit by the vehicle in the carpark of a medical centre in Berwick in Melbourne's southeast about noon on Friday.

All three were on foot but police said the two adults were not with the boy.

The Toyota 4WD was left lying on its side after also crashing into several other vehicles during the incident.

According to Victorian Police the adults' injuries are not believed to be life threatening.

The driver of the vehicle stopped at the scene and is speaking with police as the investigation into the incident continues.

Anyone with information about the collision is urged to contact Crime Stoppers on 1800 333 000.