A man jailed for life over a decades-old crime is facing a new trial after his conviction for murdering missing teenager Hayley Dodd was quashed by WA's highest court.

Key points: Hayley Dodd disappeared in 1999 near Badgingarra in regional WA

Hayley Dodd disappeared in 1999 near Badgingarra in regional WA Francis Wark faced trial two decades later after a cold-case review

Francis Wark faced trial two decades later after a cold-case review An earring found in his car was a crucial piece of evidence in the trial

Francis John Wark, 64, was sentenced to a 21-year minimum term in 2018 after Supreme Court justice Lindy Jenkins found he had killed the 17-year-old girl in the course of a sexual assault.

But in a unanimous decision, that conviction was overturned on Friday and a new trial ordered by the WA Court of Appeal.

Wark did not give evidence at his trial, but in a witness statement at the time he said he had been doing his weekly chores in Moora, including shopping and paying bills, before he left his home to travel to Perth about 20 minutes after Hayley was last seen alive.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 35 seconds 1 m 35 s Police searched Francis Wark's house in 1999 after Hayley Dodd disappeared ( ABC News )

Appeal Court Justice Andrew Beech said his conviction should be quashed.

Last spotted on a lonely country road

Hayley was last seen alive on July 29, 1999, walking along a road near Badgingarra in WA's Wheatbelt region, near where Wark owned a property at the time.

He was not charged until after a cold-case review of the evidence in 2013.

Two years later he was extradited to Perth from Queensland, where he had been serving a 12-year jail term for abducting and sexually assaulting a 31-year-old woman, whom he had offered a lift while she was walking along a remote road in 2007.

The conviction for that offence led Justice Jenkins to conclude Wark had "a propensity to pick up a lone female hitchhiker and to violently and seriously assault her so that she could not resist his sexual assault".

Decision surprises Hayley Dodd's mother

Hayley's mother, Margaret Dodd, was in the Court of Appeal to hear the decision and afterwards said she was surprised and upset by it.

"We've no choice, we're just going to be put through the mill again and that's it. It's going to be what it's going to be," she said.

Margaret Dodd says she will attend every day of Wark's retrial. ( ABC News: Andrew O'Connor )

"I'm come this far, I can't do anything but I'm not going to get unduly upset over it, even though I am upset."

Ms Dodd also vowed to attend every day of the retrial, as she did for Wark's first trial.

"It's my daughter you're talking about and I want to hear everything that goes on," she said.

Wark's lawyer, Darryl Ryan, praised the legal team behind the appeal, saying they put a lot of work into it.

Francis Wark's lawyer Darryl Ryan says he'll speak to his client about a possible bail application. ( ABC News: Andrew O'Connor )

"I haven't even had a chance to read the judgment at this point, as you can understand, but I'm sure my client will be very pleased with the outcome," he said.

Wark was not in court for the decision and remains in Acacia Prison, but he will have to appear at status conference next month when a new trial date may be set.

Mr Ryan said he would have take instructions from his client about a possible bail application, which would have to be determined by a Supreme Court judge.

Earring evidence crucial to conviction

One piece of crucial evidence was an earring discovered in a car seat cover seized from a borrowed ute Wark was driving on the day of Hayley's disappearance.

The earing sketched by Hayley Dodd's friend matched the one found in a car used by Francis Wark. ( Supplied: WA Supreme Court )

The earring was not discovered until the 2013 cold-case review, and it matched drawings done by a friend of Hayley's of the jewellery she was wearing when she was last seen alive.

Wark's lawyers suggested at the trial that the earring could have been planted by police.

But in her decision, Justice Jenkins rejected that claim and found "it was not a reasonable possibility … that it belonged to anyone other than Ms Dodd".

Wark's lawyers also argued he did not have the opportunity to abduct and kill Hayley because of the "tight timeframe" in which the crime was alleged to have occurred.

Wark has always denied having anything to do with Hayley's disappearance.