By the time officers had located the spot and found access to the top of the 50m-high Berkeley road cutting, the offenders, believed to be two young males in white shirts, had got away.

Detective Chief Inspector Michael McLean said another car was also hit by a rock, but the driver was not injured.

"Just prior to this incident occurring ... the police communications centre became aware that a number of other motorists witnessed rocks being thrown from the cutting," Insp McLean said.

"As a result, police were dispatched to the scene immediately ... before they arrived this incident occurred.

"At this stage we believe there was at least one other vehicle hit by a rock."

Insp McLean was confident the culprits would be caught as police had caught and convicted Peter Hodgkins, the Kiama Downs rock thrower who seriously injured car passenger Nicole Miller last year.

"We take all of these matters very seriously and will investigate this matter as we did that one - with vigour - and we intend to bring it to a positive conclusion," he said.

"Certainly we think that the behaviour of these persons was cowardly to say the least.

"It's incomprehensible that people would think of doing such a stupid act in this day and age."

The 56-year-old driver from Penshurst narrowly escaped death when the 2kg piece of slate suddenly smashed through the windscreen and hit him in the chest about 1pm.

"The potential here for a death or very serious injury was extremely high and I think that motorist was extremely lucky," Insp McLean said.

"For nobody to be seriously injured or killed I think we need to be grateful."

Yesterday, as officers fielded a flood of calls from witnesses to the incident, police inspectors and a Roads and Traffic Authority official visited the site from where the rocks were thrown.

The area is beyond the end of Warwick St, Berkeley, through a field and then a well-worn path surrounded by long grass and thick bush.

The precipice where the offenders stood is just 1.5m wide, unfenced and with a sheer 50m drop to the road below.

Cigarette butts littered the ground and it was clear where the offenders had gouged out pieces of slate to throw.

The RTA and police discussed the possibility of fencing off the site, but the RTA official said they were looking at "a whole lot of options".