Cheating WA farmer 'burnt wife alive'

A farmer accused of murdering his wife of 25 years in a "staged accident" by crashing a car into a tree then burning her alive repeatedly lied to police before he was finally charged a decade later, a Perth court has heard.

Gregory Paul Johnston is on trial in the West Australian Supreme Court accused of killing 56-year-old Susi Elizabeth Johnston at their Borden farm on December 6, 2008.

Prosecutor Justin Whalley told the jury on Monday that Johnston was the victim's sole financial beneficiary and also mistakenly believed she had taken out a life insurance policy days before her death.

Mr Whalley said Johnston had been unfaithful and days after his wife's death transferred $15,000 into the bank account of a woman he had met online, then married her nine months later.

The mother-of-two was allegedly incapacitated and burnt alive, while her husband had no visible injuries.

Johnston initially told police he blacked out and his wife tried to grab the steering wheel but crash experts found the tyre tracks did not show a gradual drift, rather suggesting the car had been steered deliberately.

He had also claimed he was driving at up to 90 km/h but investigators found the speed was 44 km/h, Mr Whalley said.

Police interviewed Johnston again in May 2010 and that time he did not claim he blacked out.

During another interview in February 2013, Johnston's story "changed fundamentally", claiming his wife was depressed and talked about death, Mr Whalley said.

In September 2017, he told police: "She wanted us both to go."

Mr Whalley said the suggestion Ms Johnston killed herself was not consistent with her emails, journal or personality.

Johnston's final police interview was in February 2018.

Defence counsel Linda Black pointed to the lack of direct evidence in the circumstantial case, including any proof the fire was deliberately lit.

She said even though Johnston cheated on his wife and lied to police, he was not a killer.

"It is the state's case that this passive, non-violent man burnt his wife alive in a car and stood by while it happened and then ran off," she said.

Ms Black said it was just a theory that Ms Johnston wanted to kill herself.

She said it was a strange way to take your own life but it was also a strange way for a non-violent man to kill his wife.

The trial continues.

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