THREE cowards left a woman to die on the side of the road after a sickening car crash in Sydney's west early today.

But a passer-by who held the victim's hand as she bled to death has a message for the deceased's family: "She did not die alone."

Three Asian men, who police ­believe could be illegal immigrants, are still on the run following the crash at Chester Hill about 6.30am.

The woman, aged in her 30s, was in the rear passenger seat of a Holden Commodore station wagon when it smashed into a power pole.

Witnesses told police the three men got out of the car, dragged her to the side of the road and then ran off, leaving her to die.

Police believe speed was a factor.

Heidi Karlson ran to the scene after hearing an "almighty bang" from outside her apartment.

"She was unconscious and in a ­terrible state," Ms Karlson, 41, said.

"I felt her pulse and checked her airways. I started CPR. I then felt for her pulse again but she was gone."

Ms Karlson then covered the victim, who she described as a "pretty girl", and waited for the ambulance.

"I'm going to pray for this girl. I can't believe that they could drag her out of the car and then just leave her there. I can't stop thinking if they just left her in the car, maybe she would've survived. I held her hand as she died. I just want her family and the people who love her to know that she did not die alone," she said.

The car was full of white paint, which exploded over the road.

Detective Chief Inspector Phillip Brooks, from the Traffic and Highway Patrol Command, said the incident is shocking.

"Not only is this a tragic event, it is also cowardly that they would leave a young lady without providing first aid before fleeing the scene," Insp Brooks said.

"This death takes this years road toll up to 186 people."

Soon after the incident police knocked on the door of the car's registered owner, who lives in Lidcombe.

He was not home, so police are continuing to search for the trio. Police believe the car was not stolen.

"From the amount of household paint in the car it's a possibility that they are a painting crew," Inspector Darryl Hawes said.

"They obviously give little regard for human life.

"There is a possibility that they are illegal immigrants doing cheap work.

"However, we have not confirmed the identification of anyone."

The female victim was wearing a jacket, long pants and work boots.

Police are reviewing CCTV footage from the scene.