The Queensland Government has doubled the reward for information about the disappearance of Mackay teenager Marilyn Wallman, in a bid to finally solve the state's longest-running child abduction case.

The 14-year-old was last seen on March 21, 1972 while riding her bike to school.

Police have long believed she was kidnapped and murdered and are now offering a $500,000 reward to anyone who can help find her killer.

They have identified a person of interest, and are appealing for anyone who may have knowledge of what happened to come forward — 46 years later.

Lead investigator Detective Senior Constable Cindy Searle said the person still resided in the Mackay region, but officers were still missing a piece of the puzzle.

"We feel we are very, very close to an answer but we need a bit more assistance from the public," she said.

"I believe someone, somewhere knows something they can bring to us.

"Our message to them is please come forward … the family has lived for 46 years with not knowing what happened to their daughter and we would like to give them those answers."

Marilyn Wallman's school bike and bag were found on the side of the road. ( Supplied: Queensland Police )

Detective Inspector Damian Hansen said police hoped the passage of time would encourage witnesses to reveal what they know.

The local Mackay newspaper two days after Marilyn went missing. ( ABC News: Sourced State Library )

"Loyalties deteriorate over that time, marriages breakup, and people do talk," Detective Inspector Hansen said.

"Who they were loyal to 46 years ago may not be who they're loyal to now.

"I just ask them to listen to their conscience.

"It's a 14-year-old girl — come forward."

Police Minister Mark Ryan said the reward for information leading to a conviction had been increased from $250,000 for a period of 12 months only.

"Sometimes the community needs an added incentive to come forward with this information," he said.

"That's why we've taken this extraordinary step to boost the reward — the first time we've actually boosted a reward.

"I've been advised also that this is the largest government reward ever offered for a case of this type in Queensland."

Family haunted by disappearance

Marilyn's brothers found her bike and school bag beside the road near a cane field shortly after she vanished.

Earlier this year Rex Wallman told the ABC he still had vivid memories of that day, and he was haunted by what happened to his sister.

"It was pretty traumatic ... it's like it just happened yesterday," Mr Wallman said.

"Mum needed medication because she was so distraught and Dad was out every day searching."

In 2015, DNA testing confirmed that a partial skull found in 1974 belonged to Marilyn.

Further searches of the area in 2016 failed to find any further remains.

An aqua and white Holden sedan seen on the road where Marilyn went missing also remains part of the investigation.