On March 21, 1972, the 14-year-old schoolgirl left her Mackay home 10 minutes before her two brothers. A few minutes later, following the same route, her brothers discovered her bike abandoned on the road, her school books scattered.

Two years later a bone fragment was found that, in 2001, was confirmed to belong to Marilyn.

A $250,000 reward was issued for information over Marilyn's murder, which on Tuesday Police Minister Steven Miles announced was doubled to Queensland record of $500,000.

"Justice is not extinguished by the passage of time and we're here today to give a voice to those who no longer have a voice," he said.

"We want to get to the end of this story, we want to provide justice not only for Marilyn but also her family."

Homicide Detective Inspector Damien Hansen said the increased reward will remain in place for 12 months.

It will only apply to information that leads to the arrest and conviction of any person or persons responsible for Marilyn's murder.

He said the time was right after an extensive review of the cold case to increase the reward in hopes of encouraging people with knowledge of Marilyn's disappearance to come forward.

Loyalties over the years may have changed and people who previously had kept silent might decide to speak, Inspector Hansen said.

"I'd be appealing for any person who has knowledge of this crime to contact Crime Stoppers," he said.

"It's 46 years since this has occurred, people aren't getting any younger and we'd seek them to come forward and listen to their conscience."

Inspector Hansen said police had identified a person of interest and due to the extensive passage of time, police were aware that any possible arrest following the reward's increase could lead to a long court case.

"It's getting more and more difficult ... if we were to make an arrest now, it may well be fifty years," he said.

"It's that crunch time for the investigation.

"We're talking an innocent 14-year-old girl on her way to school who we believe was murdered."