A mother who had a gun pressed to her head before being pistol-whipped in the face in a terrifying carjacking thought she was "going to die now".

The disability worker was driving home from work on Harrington Drive, Narre Warren South, when an SUV carrying three men rear-ended her just after 9pm on Thursday.

The woman, Kay (who did not use her surname), got out of the car to exchange details with the driver. But instead of swapping insurance company details the man produced a gun, pressed it against her forehead and demanded her keys.

In shock, Kay, 57, asked "what for", before she was struck in the face with the butt of the pistol. She then watched two of the men flee in her car.