The mother of a young learner driver who was killed in a crash in Sydney's northwest has spent the night at the bedside of her critically-injured husband.
The two were involved in a collision between a Suzuki Swift and Falcon sedan at Glenorie about 4.50pm Friday, which killed the 19-year-old at the scene.
The girl's father, aged 58, was supervising her at the time and was taken to Royal North Shore Hospital in a critical condition with multiple injuries.
A hospital spokesman told AAP the man remained in a critical condition on Saturday afternoon.
Chief Inspector Frank Gilroy said the incident was a reminder for all drivers to take care on the road.
"(It's) absolutely tragic when these things occur," he told reporters on Saturday.
"Naturally mum and wife was absolutely devastated. She's been at her husband's bedside since last night."
Several witnesses attempted to help the victims and have since spoken with police.
One of them, David King, said he tried to talk to the pair to keep them conscious through the car door.
"You couldn't open the door, you couldn't do anything," he told the Seven Network.
"She died right in front of me."
Mr King said he rushed down to help as soon as he heard the crash.
"She's about the same age as my daughter, so that hit hard. Too young."
Police are investigating whether the teenager driver was attempting a U-turn when the cars collided.
The driver of the other car, who was not believed to be seriously injured, has undergone mandatory testing.