A 30-year-old Tarneit woman was flown to hospital with life-threatening injuries after she came out of a moving car on the Princes Highway on Sunday.

Victoria Police said the woman was travelling in a Honda sedan heading outbound on the freeway before Little River about 11.30am.

A 28-year-old Tarneit man driving the car stopped at the scene where he has spoken with police while the woman was flown to hospital in an air ambulance.

Police said the exact circumstances of the incident were yet to be determined, with the investigation ongoing.

A spokesman for Ambulance Victoria said the woman was flown to The Alfred in a critical condition with head and other injuries.

The incident forced the Princes Freeway to be closed outbound with the traffic bumper-to-bumper across all four lanes in some sections, including the emergency lane, and all vehicles were being diverted off the freeway at the Werribee exit from early in the afternoon.

VicTraffic said Melbourne-bound lanes were also slow as police were allowing cars to make U-turns and head back towards the city due to the delays on the freeway.

Motorists were advised to use Ballan Road or Geelong-Bacchus Marsh Road to avoid the freeway as there is no timeframe available for when it will re-open.