A six-year-old boy is in a critical condition after the car he was in and a concrete truck collided on the Hume Motorway in Sydney's south-west, just before midday on Thursday.
The boy suffered head injuries and a broken leg when he was thrown 10-metres from the car, a Toyota Tarago.
The collision occurred on the south-bound lanes of the highway at Mount Annan, about 50 metres past the Narellan Road turn-off.
Police believe the Tarago had stopped in the highway's break-down lane when it was struck from behind by the truck.
The truck driver was trapped in the cabin of his vehicle for more than an hour.
The Tarago was carrying the boy, a girl and two women.
A NSW Ambulance spokeswoman said the boy suffered a fractured leg and head injuries.
The motorway's south-bound lanes have been closed at the crash scene and two rescue helicopters have landed on the road.
The boy was being stabilised at the scene and would be flown to the Children's Hospital at Westmead, the NSW Ambulance spokeswoman said.
The truck driver was also expected to be flown to hospital, the spokeswoman said, although the extent of his injuries was not immediately known.
The two women from the Tarago were taken to Westmead Hospital by road ambulance with leg and abdominal injuries.
The Transport Management Centre said the Hume Motorway was closed south-bound at Narellan Road, and motorists should avoid the area or expect significant delays.
Traffic was banked up about five kilometres at 1pm.
South-bound motorists are being diverted onto Narellan Road and are advised to travel via Appin Road, Wilton Road and Picton Road to rejoin the motorway.
Heavy vehicles should consider delaying their journey as the alternative route is not suitable.