The children, aged 1 and 2, were strapped into booster seats in a white four-wheel drive heading south on the Princes Motorway around 1pm, when the car clipped another vehicle, lost control, and flipped several times.

The 4WD, with a father, mother and the two children inside, came to rest on the right-hand shoulder of the road.

Ambulance NSW duty commander Inspector Norm Rees said the mother was taken by ambulance to Wollongong Hospital with back injuries, while the two children were airlifted to Sydney Children’s Hospital with head injuries.

It is thought one of the children also lost consciousness during the incident.

‘‘It looks like the 4WD hit the back of another car, and lost control,’’ Insp Rees said.

‘‘One child had head injuries, and the other was less severely injured. After being assessed by a doctor, that child will go to hospital by helicopter also.’’

The driver of the other car was said to have suffered only shock. The children’s father was not seriously hurt and was taken to hospital to be with his children.

Scores of police, ambulance and fire crews responded to the incident, with rescue trucks from as far as Engadine on the scene.

The crash closed the road southbound, turning the normally busy motorway into a virtual parking lot. Traffic banked up for at least five kilometres in the baking afternoon heat, with temperatures reaching at least 40 degrees on the road.

Several drivers lay down in the shadows under their cars, rather than sit in their rapidly heating vehicles.

Traffic was eventually diverted onto the Princes Highway at Helensburgh, but the road remained closed through much of the afternoon as crash scene investigators collected evidence.