The driver of a car linked to a fatal hit-and-run in Brisbane has been found dead.

Ashleigh Humphrys died after being struck by a car near the Toowong roundabout about 3:50am on Sunday after spending the night celebrating her 20th birthday.

The initial investigation indicated Ms Humphrys had been intoxicated and lying on the road or on her hands and knees when she was hit.

Police appealed for the driver of a dark sedan seen in security footage around the time of the incident mounting the footpath and swerving back onto the road to come forward.

A car was seized on Monday and police began scientific testing on it.

The driver of the car was found dead yesterday, police said.

The 57-year-old man had been helping officers with their inquiries.

Police said there were no suspicious circumstances surrounding his death.

Driver may not have known girl had been hit: police union

Queensland Police Union president Ian Leavers said the driver who hit Ms Humphrys may not have known what occurred.

Mr Leavers described both deaths as an "absolute tragedy".

"I am aware that there would be quite easily a perception that someone should come forward," he said.

"There may be information to suggest the driver didn't even know that this occurred.

"We now have two families in mourning. There are two families who will never be the same as a result of this.

"Obviously both of these matters will now be coronial investigations before the coroner.

"I think it's very important that we should never prejudge things that occur."

Last weekend, police said they received a phone call prior to the hit-and-run from a member of the public who said he had concerns for the safety of a woman he had seen walking on Mount Coot-tha Road.

They said the caller told police the woman looked to be affected by alcohol or another substance "and she was either a danger to herself or other road users".

The investigation into the accident is continuing.