The collision occurred about noon on Monday, March 18, in the southbound lanes at Braemar, about 45 minutes after a more minor two-vehicle crash in the same vicinity.

Police had closed one southbound lane as the earlier crash site was being cleared and were still diverting traffic into one lane when a truck crashed into the back of a hatchback, forcing it into the rear of a station wagon.

Police said a 69-year-old front seat passenger of the hatchback, a woman from Cootamundra, was trapped for some time before emergency service workers freed her.

She was flown to Liverpool Hospital in a serious condition with a chest injury, a NSW Ambulance spokesperson said.

Police said she was also reported to have suffered fractures.

The 69-year-old male driver of the hatchback, also from Cootamundra, sustained a head injury, a NSW Ambulance spokesperson said, and was taken to Liverpool Hospital in a stable condition.

Police said he would also undergo mandatory testing.

The male driver of the truck was uninjured and was taken to Bowral Hospital for mandatory testing.

On Monday afternoon he was in police custody at Southern Highlands police station in Moss Vale.

The couple travelling in the station wagon, both aged 68, were also uninjured but the male driver has undergone mandatory testing.

Officers from the NSW Police Force Crash Investigation Unit are examining the scene.

The southbound lanes of the motorway remained closed as of 6pm.

Traffic is being diverted onto the Old Hume Highway via Mittagong.