Harmony Bryant, 26, was found on the brink of death in bushland near Bonny Hills, about 25km south of Port Macquarie in NSW on August 16, 2003.
Sustaining a broken pelvis, a fractured wrist and left arm, and burns on 60 per cent of her body, she was quickly taken to hospital while her car, a Hyundai Excel, was found on fire about 200 metres from the road.
She spent a month in hospital before dying from her injuries on September 19, 2003.
Ms Bryant's death has plagued detectives for years since.
Evidence indicates she fell off a nearby cliff before being brought back to her car by an unknown person or group of people.
A 2011 inquest also found she had died from burns caused by a person or persons unknown.
Police found she had booked accommodation for two people at a nearby caravan but a second person has never been identified and no one has ever been charged over her death.
The NSW government and police in 2011 offered $100,000 for information about Ms Bryant's murder.
But on Thursday, this reward was increased to $1 million as police reinvigorate calls for public assistance.
After her discovery, police initially believed the 26-year-old was the victim of a car crash.
Her death would have continued to be treated as non-suspicious if her family had not conducted their own investigation into the incident, according to the coroner's 2011 report.
The officer in charge had made his incorrect assessment despite many unusual features, the coroner found, and did not change his report even after receiving information suggesting something was amiss.
As a result, opportunities available in the "vital" 48 hours after she was found had been missed, the coroner said, as he referred the case to the homicide squad.
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