Frank and Karen Giufre have described the helplessness of watching their daughter in intensive care after she was dragged several metres by a car at high speed in Sydney's west six weeks ago.

Police have no idea who was driving the car or why the driver failed to stop as Ms Giufre screamed and was dragged along Myall Road at Casula on September 7.

"I can't help her, there is nothing I can do to help her," her mother said through muffled tears as she held her husband's hand.

"We don't know whether she will talk, or see or how she will end up."

Mr Giufre has watched the horrific footage of his daughter clinging to the rear passenger window of a silver sedan as she was dragged along the road.

When she was finally released from the car, her skull cracked on the ground.

She was left for dead in a gutter near the intersection of Ingham Drive and found by a passing black car about 6pm.

The 18-year-old remains in Liverpool Hospital with severe brain damage. She is blind in one eye and unable to communicate with detectives to explain what happened.

The Giufres spend 24 hours a day with their daughter, giving up their jobs to do shifts by her bedside.

"Basically the image of Samantha that we first saw in the hospital is stuck in our head. Personally I don't know how you get it out of there; I just want to see her back the way she was," Mrs Giufre said.

The teenager was described as an athlete who played soccer at an elite level and who had many close friends.

"Knowing someone has taken that away from her ... she's just a good girl, she doesn't deserve this," Mrs Giufre said.

Her parents say she had left their house with her cousin to visit friends at Casula.

"We had our daughter at home on Father's Day, had a nice day there till about 5 o'clock when she decided to go see a friend at Casula," Mr Giufre said.

Now, her parents are begging the people in the car that dragged their daughter to help them understand what has happened.

"We'd like to make a plea - someone must have seen or saw something, some yelling or screaming or maybe the car," her father said.

"We don't know how she came to be where she was, what's happened to her, but it is out there now and I'm sure someone has seen that car around," Mrs Giufre said.

Liverpool Police Detective Inspector Dean Johnstone said police were at a dead-end and urged anyone who knew what had happened to contact police.

"This incident has left a very young woman with very serious head injuries," Detective Inspector Johnstone said.

"I have no doubt people have knowledge about this incident and we are calling for those people to come forward.

"This is a very cowardly and callous attack."

Police are looking for the driver of the car and believe there was at least one other passenger inside.

"We hope someone out there can help us," Mr Giufre said.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.