A convicted rapist has been found guilty of murdering missing 17-year-old Hayley Dodd, more than 18 years after the girl vanished while walking along a remote road in Western Australia's Wheatbelt region.

Francis John Wark, 61, was found to have lured the teenager into the borrowed ute he was driving on July 29, 1999, and killed her in the course of a sexual assault.

Wark owned a property in Badgingarra, near where Hayley was last seen walking along North West Road.

Her body has never been found and her family and friends never heard from her again.

Many of them packed the Supreme Court for Monday's verdict, some wearing yellow flowers to honour Hayley and all other missing people in Australia.

Margaret Dodd and supporters leave the Perth Supreme Court wearing yellow flower crowns in honour of Hayley. ( ABC News: Emily Piesse )

They all sat silently as Justice Lindy Jenkins delivered her guilty verdict.

In her 200-page decision, Justice Jenkins said it was a "sad but inescapable" conclusion that Hayley was dead and that Wark was responsible.

"I am satisfied beyond a reasonable doubt that the accused [Wark] picked up Ms Dodd and killed her on 29 July, 1999 by some unknown means," Justice Jenkins said.

Wark was not charged with Hayley's murder until 2015, when he was serving a 12-year jail term in Queensland for physically and sexually assaulting a 31-year-old woman he had picked up on a remote road in the state's north.

Justice Jenkins, who presided over Wark's seven-week trial last year, said based on evidence about the Queensland crime she was satisfied 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".

Single earring the vital clue

A crucial piece of evidence during his trial was the discovery, during a 2013 cold case review, of a single earring embedded in the fabric of a car seat cover seized from the ute in 1999.

The earring matched drawings done by Hayley's friend, of earrings she had bought two days before she vanished, and was believed to have been wearing when she was last seen alive.

Wark's laywer Darryl Ryan had suggested the earring may have been planted by police, who were under pressure to solve the crime, or that it could have come from someone else who had been in ute before Hayley's disappearance.

Composite of the earring sketched by Hayley Dodd's friend, and the one found in a car used by Francis Wark. ( Supplied: WA Supreme Court )

However Justice Jenkins rejected both of those submissions.

"It is not a reasonable possibility that the earring found on the seat cover was left there prior to 29 July, 1999, or that it belonged to anyone other than Ms Dodd," Justice Jenkins said.

Wark's trial was also told that during his Queensland crime, he had demanded the victim give him an earring, telling her "I want to keep it."

Justice Jenkins said that evidence along with the discovery of Hayley's earring led her to find "the accused [Wark] is a person who would be likely to take an earring as a trophy from his victim".

Another vital piece of evidence at Wark's trial was the DNA analysis of a single 18-millimetre strand of hair, found during the cold case review, in the debris taken from the ute.

Mr Ryan had highlighted that male DNA was also found on it, meaning there was a possibility it had not come from Hayley.

Justice Jenkins appeared to agree, finding that while it was possible it was Hayley's hair, she was "not persuaded" that it was.

Prosecutors had argued Wark should be guilty of wilful murder arguing he intended to kill Hayley, but Justice Jenkins said she was not satisfied beyond a reasonable doubt that was the case.

"That is because I find that it is a reasonable inference to draw from the evidence that the accused picked up Ms Dodd in order to sexually assault her and that he killed in the prosecution of that unlawful purpose, without necessarily forming an intention to kill her," Justice Jenkins said.

The crime of wilful murder no longer exists in Western Australia, but Wark was tried under the laws that were current in 1999, when wilful murder was still distinguished from the crime of murder.

Wark has always denied anything to do with Hayley's disappearance, claiming that at the time he was doing his weekly "ritual" of shopping in Moora, about 50 kilometres away from Badgingarra.

Justice Jenkins said while the timeframe of the murder was "extremely tight", Wark did have "the opportunity" to "encounter" Hayley on his journey back to his property, murder her and dispose of her body.

Wark will face a sentencing hearing next week, to allow Justice Jenkins to determine what minimum term he should serve before being considered for release on parole.

Family still searching for body

Margaret Dodd campaigned for 18 years to get justice for her daughter and her family, but was too upset after the verdict to comment.

Margaret Dodd (in black), mother of missing teenager Hayley Dodd, was flanked by family and friends as the verdict was read out. ( ABC News: Emily Piesse )

Hayley's sister Toni Dodd said the family did not think this day would ever come, but they now wanted Wark to tell them where Hayley's body was.

"Tell us where Hayley is so we can put her to rest. Give our family some peace."

"We didn't just lose a sister, we lost parents as well," Ms Dodd said.

She said the family would always support changes to the law so that killers who do not reveal the location of their victims' bodies do not get parole.

Ms Dodd said it had been "horrible" sitting through the seven-week trial last year.

"We had to sit through it to know every detail," Ms Dodd said.

"We want to know what's happened, to try to figure out why, try to figure out if there are any clues as to where she is and why did he do it."