Mystery surrounding the tragic death of Andrew Symonds has deepened after the former Australian cricketer's family revealed they did not know why he was driving alone late at night.

The 46-year-old was killed on Saturday night when his vehicle veered off the road and rolled over just outside Townsville. Bystanders who heard the crash rushed to help, but were unable to save him.

Symonds' distraught sister Louise visited the scene of the tragic accident on Sunday, leaving a handwritten note expressing her sense of loss.

"Gone far too soon!" the note read. "Rest in peace Andrew.

"I wish we had one more day, one more phone call.

"My heart is broken. I will always love you my brother."

But Speaking to Daily Mail Australia on Monday morning, Louise confessed she doesn't know what he was doing on the road that night.

"We just don't know," she told the publication.

"It's just awful."

Channel 9 reporter Mia Glover told FIVEaa radio on Monday morning she can "see that there's a lot of brake marks on the road from where his car veered off before it eventually rolled".

"At this stage police still have no idea why he left the road," she added.

"One rumour that's going around by locals is he may have swerved to miss an animal but everything's under investigation at the moment."

Queensland Police are investigating the crash and will prepare a report for the coroner.

A witness, who found Symonds in his vehicle after he crashed, told the Courier Mail the cricketer's two blue heelers refused to leave his side.

"One of them was very sensitive and didn't want to leave him," she said. "It would just growl at you every time we tried to move him or go near him.

"My partner tried to get (Symonds) out of the car, to put him on to his back.

"He was unconscious, not responsive and had no pulse."

Waylon Townson tried to help Symonds but there was nothing he could do.

"He was stuck in there, so I tried to pull him out," he told Nine News. "(I) started doing CPR and checked his pulse but I didn't get much response."

Symonds' death is another devastating loss for the Australian cricket community, which had already farewelled fellow champions Shane Warne and Rod Marsh earlier this year.