A husband accused of killing his wife on their farm in WA's Great Southern had been having an affair with another woman and transferred tens of thousands of dollars into her account in the days after his partner's death, a court has been told.

Key points: Susi Johnston was allegedly murdered in a staged crash on a WA farm 10 years ago

Susi Johnston was allegedly murdered in a staged crash on a WA farm 10 years ago Nine months on, Gregory Johnston married a woman he had been having an affair with

Nine months on, Gregory Johnston married a woman he had been having an affair with Mr Johnston's lawyers say there's no proof he staged the crash or set the car alight

Gregory Paul Johnston was charged last year over the death of 56-year-old Susi Johnston in December 2008, when a car she was a passenger in hit a tree on the Borden property.

The vehicle caught fire while Ms Johnston was still inside.

Mr Johnston escaped uninjured and is now on trial accused of her murder.

The jury in the trial will be shown excerpts from Ms Johnston's diary before her death. ( Supplied: WA Police )

Prosecutors allege he staged the accident after somehow rendering his wife incapacitated, before setting the car on fire.

A triple-0 call made by Mr Johnston shortly after the incident is set to be played to the jury during the trial.

Johnston's distress 'simulated and fake': prosecutor

Prosecutor Justin Whalley said the state would suggest audible sobbing from Mr Johnston on the call was "simulated and fake".

"How is it the accused man got out without a scratch and Susi Johnston was trapped and incinerated?" Mr Whalley asked.

"This was a staged accident."

Mr Whalley told the court that despite telling police his marriage was "pretty good", Mr Johnston had been having an affair with a woman he had met online.

Gregory Paul Johnston escaped the crash without injury, the court heard. ( ABC News: Frances Bell )

He said the accused had paid the woman $1,000 for a back massage and had also transferred $500 into her account on another occasion.

Less than two weeks after the car crash that killed his wife, Mr Johnston is said to have withdrawn $15,000 to give to the woman.

The pair married in Sydney nine months later.

The court was also told Mr Johnston had attempted to make a claim on a life insurance policy he believed his wife had taken out in the week before her death.

No evidence to back prosecution's theory, defence argues

Mr Johnston has pleaded not guilty, and his lawyer Linda Black argued there was no evidence to back up the theory her client had staged the accident and set the car alight.

Defence lawyers will argue Mrs Johnston grabbed the car's steering wheel before the crash. ( Supplied: WA Police )

Ms Black conceded her client had lied to police about his affair, but said he had no criminal record and had never been violent towards his wife.

She said the prosecutor's case that Mr Johnston was a "passive, non-violent man" who "burnt his wife alive" was implausible.

"What kind of man does that to his wife of 25 years?" Ms Black asked the jury.

"He was a cheating spouse, he had affairs … but is this a man who would set his wife alight?"

The defence is set to argue Ms Johnston grabbed the car's steering wheel before the crash, causing it to veer into the tree, possibly in an attempt to take her own life.

The jury will be presented with excerpts from Ms Johnston's diary to give them an understanding of her mental state in the days and weeks leading up to her death.

Neighbours find husband 'sobbing' on kitchen floor

The Johnston's neighbours, Judith and Michael Summers and their son Ashley, were the first at the scene of the fiery wreck, having spotted smoke coming from the adjacent property.

Mr Summers and his son did not see any movement inside the car, and tried to extinguish the blaze using a water tank on the back of their farm ute before going to the Johnston's homestead.

There they found Mr Johnston lying on the kitchen floor.

Mr Summers told the court while his neighbour appeared uninjured, he was in a highly distressed state.

He said he knelt down beside Mr Johnston and asked how he was.

"He was terribly upset, sobbing," Mr Summers said.

"There was a box of tissues on the table. I got them down, he was wiping his eyes."

He recalled talking to Mr Johnston about that year's harvest in an attempt to keep him calm.

"Another thing he told me was that he thought he might have blacked out somewhere on the road," Mr Summers said.

The trial has been set down for five weeks.