The collision occurred at Braemar, near Mittagong, at 11.15am on Monday, March 18, about 45 minutes after a more minor crash in the vicinity.

Police had closed one southbound lane following the earlier crash as the site was being cleared, when a truck smashed into the rear of a hatchback, forcing it into the back of a station wagon.

The collision left two elderly people in the hatchback with fractures, head and chest injuries.

The uninjured 55-year-old male truck driver was arrested and underwent mandatory testing, before being charged with a string of offences.

"The driver was charged with dangerous driving occasioning grievous bodily harm, cause bodily harm by misconduct in charge of motor vehicle and negligent driving occasioning grievous bodily harm," a NSW Police spokesperson said.

The two injured people in the hatchback are from Cootamundra in the South West Slopes of NSW

A 69-year-old female front-seat passenger in the car was trapped in the smashed car for some time after the crash.

She sustained fractures and chest injuries and was airlifted to Liverpool Hospital in a serious condition.

The 69-year-old male driver in the car had a head injury and was taken to Liverpool Hospital in a stable condition. He will undergo mandatory testing.

There were no injuries in the earlier crash on the Hume Motorway.

Officers attached to the Crash Investigation Unit will investigate the incident.

The truck driver was was granted conditional bail to appear before Moss Vale Local Court on April 16.