Police say a 21-year-old woman from Kiama Downs was turning right onto the Princes Highway from Woollybutt Drive when her car and another vehicle collided about 5.50am on Thursday.

The driver of the second car, a 61-year-old man from Wyong, allegedly rammed the rear of the woman's car further along the highway, after she failed to stop and exchange details.

Lake Illawarra police crime prevention officer Mark Scott said the impact pushed the woman's car into the centre median fence.

"As a result of the impact, it appears that a 20-year-old female passenger from Oak Flats, in the rear seat of that car, was ejected through the rear window," he said.

The woman landed on the opposite side of the highway and was nearly struck by a car coming down the northbound on-ramp.

The driver, Amanda Coleman, pulled over when she saw a truck driver frantically waving to draw attention to the injured woman.

"She had [been thrown] across three lanes," Ms Coleman said.

"If the truck driver didn't wave me down I wouldn't like to think what would have happened."

The woman sustained a major head injury and other minor injuries.

A NSW Ambulance spokesman said she was taken to the NSW Ambulance Rescue helicopter base at Albion Park and flown to St George Hospital, where she underwent surgery.

A spokeswoman for the hospital said the woman was in a critical but stable condition late on Thursday.

The incident is being investigated by Highway Patrol officers from Lake Illawarra police.

Witnesses should call Lake Illawarra Police Station on 4232 5599 or Crime Stoppers: 1800 333 000.