Police allege the 50-year-old woman had parked on Progress Avenue in Eastwood at 12.40pm on Wednesday when she tried to reverse her car and hit two other cars behind her.
The girl had been crossing between these parked cars and was hit and trapped between them.
Emergency services including police, CareFlight and NSW Ambulance arrived at the scene, where they found the girl with injuries to her pelvis.
It is believed the girl was trapped for three minutes. A doctor and intensive care paramedic from CareFlight treated her after the helicopter landed in the adjacent Eastwood Park.
She was taken to Westmead Children’s Hospital in a serious condition.
Police allege the woman behind the wheel attempted to leave the scene and was found a short distance down the road. She was given a breath test by police, which allegedly returned a reading of .303. The legal limit is 0.05.
Inspector Anthony Boyd from Ryde Local Area Command said the girl’s condition was initially critical before it was downgraded to serious but stable.
“The offending driver in this instance was heavily impaired by alcohol, with an alcohol reading approximately six times the legal limit,” he said.
“Obviously with that level impairment, this could have been a catastrophic incident involving a young lady which could have resulted in her death or, in fact, others. With that level impairment, how do you get behind the wheel of a motor vehicle and attempt to drive, let alone walk?”
Mr Boyd said a total of 7 vehicles were involved in the incident, which took place in a street adjacent to a small suburban shopping centre where scores of people were around.
“It could’ve been a hell of a lot worse,” he said.
The woman was arrested following the breath test and taken to Ryde police station, where she will be interviewed by crash investigators.
Police expect charges will be laid on Wednesday night.
Investigators remained at the scene, which was declared a crime scene, on Wednesday afternoon. Two other cars were also damaged in “a separate incident” with the same car, police allege.