The news comes after the Mandurah Mail released a four-part podcast, Annette: Cold Case Unlocked, which explores the mysterious disappearance and death of the bubbly, carefree 19-year-old girl, from outside the local post office in 1980.

The Mail has campaigned for additional resources to be applied by authorities and police, with the series highlighting issues with the case and the fact there has never been a reward for information.

WA Police Minister Michelle Roberts and Mandurah MP David Templeman met with Annette Deverell's mother Margaret Carver on Monday after the reward was approved by politicians at a cabinet meeting in Pinjarra.

On Saturday, September 13, 1980, Annette partied with her friends at the pub, left to buy a packet of cigarettes and vanished.

Initially, police treated Annette's disappearance as a missing person's case. But her friends and family had no reason to think she would run away.

PODCAST EPISODES:

Two years later, their worst fears were realised when her skull and bone fragments were found in Pinjarra bushland, about 30 kilometres from where she was last seen.

Almost four decades later, Annette's family still crave answers.

An inquest into her death has never been held and a reward for information has never been offered - until today.

Mrs Roberts said the cabinet voted unanimously to establish the reward.

"This will enable us to just have a focus on Annette and for police to be able to monitor any calls that come in and follow up on leads if they get any," she said.

"In every instance more than just the offender knows what's going on."

And in an extraordinary move, the government may offer immunity from prosecution, provided the person that comes forward was not directly responsible for Annette's death.

"Someone who may have been connected or complicit in some way, providing they are not the person who killed her," Mrs Roberts said.

Mrs Roberts said aspects of Annette's case were strange, including the man Annette was seen with outside the Pinjarra Road post office on the night she disappeared, described as a "surfy-type blonde", who has never been identified.

"He may or may not have something to do with it, but someone surely must be able to identify who that was," she said.

"That's a bit incomprehensible.

"There was also a firearm located in the vicinity of her (remains) and police cannot say if the firearm has anything to do with it or not."

Mrs Roberts said WA Police were retesting exhibits for DNA with improved scientific processes.

"We've got a whole cold case review team going over cold case after cold case and retesting any exhibits that they've got," she said.

Mr Templeman said the Mail's podcast series and coverage were crucial in putting the case back in the spotlight.

"From a local perspective, the stories and podcast have been very interesting," he said.

"It's been important, obviously it's been 40 years. It's prompted a lot of localised interest."

Mr Templeman said the reward was an "important step" in assisting police to solve the case.

"After the localised story, I raised the issue of a reward with Michelle, who understood the importance of the matter for Mandurah and Annette's family," he said.

"We want to make sure all opportunities are explored.

"I can only imagine what it would be like for a parent who lost a loved one in this way and we don't know who was responsible."

Mrs Carver thanked the state government.

"I'd like to thank David for pushing so hard for this and Michelle for organising the reward," she said.

"I just hope that with this reward someone will come forward and solve this case after all this time."

Annette's brother Michael "Digger" Deverell said he wanted the case to be solved before his mother Margaret passed away.

"It needs to be solved before then," he said.

"It also needs to be solved before the people involved die. If this case is ever going to be solved, it will be after this podcast."

Annette: Cold Case Unlocked has had more than 60,000 plays since its launch in September, which coincided with the anniversary of Annette's disappearance on September 13, 1980.