The 19-year-old woman tumbled out of a northbound car and into the path of a truck about 7.45am Friday.

Lake Illawarra Police Commander Superintendent Wayne Starling said the truck driver, also travelling north, could not avoid the collision.

“[The car] was seen to swerve on the roadway and a short time later a female has left the vehicle and been hit by another vehicle and died immediately,” he said.

“Several vehicles took evasive action to try to avoid hitting the female, unfortunately one couldn't - just didn't have sufficient time.”

The woman, from Sydney’s south-west, was a passenger in a car being driven by a 20-year-old man from Barrack Heights.

The police investigation now centres on what could have compelled the woman to leave the car while it was still moving at speed.

Both drivers were taken to hospital for mandatory blood and alcohol testing.

The accident occurred between the Fowlers Road and Emerson Road overpasses and closed northbound lanes of the motorway for more than five hours.

As a long queue of traffic formed to the south, a few drops of rain fell on the cordoned-off crash site.

NSW Ambulance paramedics treated several witnesses for shock, including an off-duty police officer who was the first to come across the accident.

“It’s horrific for police, but even more so for the family and friends," Superintendent Starling said.

"It’s horrible for the whole community; it will send a ripple effect through the whole community.”

Superintendent Starling said the relationship between the woman and car’s male driver was still being established.

“Extensive inquiries are under way and will continue throughout the day.”

Truck drivers travelling behind the one involved in the collision also struggled to take evasive action.

Crime scene and crash investigators remained the crash site for several hours.

Southbound traffic was unaffected.

Police have called on anyone who witnessed the crash to contact police. 

Contact Crime Stoppers on 1800 333 000.