The rock hit the 56-year-old in the chest before ricocheting to his left wrist.

He was forced to drive for almost a kilometre in wet conditions, clutching his chest while trying to steer to safety his car, carrying his wife and two other passengers.

The attack happened about 1pm at the Berkeley cutting, just before the Northcliffe Dr turn-off, heading south.

Acting Sergeant Marty Weston said a motorist travelling behind the victim's dark green hatchback sedan reported seeing a boy, in the company of another, drop the rock from the 50m cutting.

"From what we can gather, they have been there five or 10 minutes before the incident," Sgt Weston said.

He said the impact of the rock caused the driver to clutch his chest while trying to negotiate the busy freeway, colliding with the guard rail before bringing his car to a halt.

He was taken to Wollongong Hospital, where he was treated for abrasions before being allowed to leave.

Illawarra ambulance district manager Norm Rees said the 56-year-old was lucky to escape serious injury.

"If it had been level it would have hit him in the face," Mr Rees said.

"If it had hit the back screen, it would have been the same result as Nicole Miller (critically injured in a rock throwing accident at Bombo last year).

"The kids just don't realise what they are doing. They have the potential to kill someone."

The two children observed at the peak of the Berkeley cutting were described as wearing white T-shirts.

A crime scene was established at the cutting.