A man who used his commando training to stab his former partner to death in an act of "outrageous violence" inside a Perth court complex has been sentenced to life in jail with a 24-year minimum.

Key points: Paul Turner stabbed Sarah Thomas to death during a mediation session

Paul Turner stabbed Sarah Thomas to death during a mediation session The couple had been arguing over $2,000 Turner claimed he was owed

The couple had been arguing over $2,000 Turner claimed he was owed He had been taught how to fatally stab people by targeting their major arteries

Paul Gary Turner, 43, was found guilty by a Supreme Court jury of murdering the 33-year-old mother of his two children, Sarah Thomas, during a mediation hearing at the Joondalup courthouse in December 2016.

The hearing was to discuss a claim by Turner that Ms Thomas owed him about $2,000.

He stabbed her through the throat with a knife he had smuggled into the meeting, when she told him she would not give him any money.

Prosecutors said Turner specifically chose a public location to carry out his act for maximum impact.

"It was quite outrageous violence in a public place by a person who took the opportunity to make a public statement," prosecutor James Mactaggart said.

Sarah Thomas, 33, had separated from Turner several months before she was murdered. ( Facebook: Sarah Thomas )

The events played out in front of a court officer who was mediating the hearing, when he went to open the door.

It left staff in the courthouse traumatised and one had been unable to work since.

The events before and after were recorded by CCTV cameras and show Turner waiting to go inside the meeting and then walking out and slumping to the floor in the aftermath.

Ms Thomas suffered six wounds, including a fatal stab to the neck that severed her carotid artery.

The court was told years earlier Turner had undergone training in "commando knife skills", which included being taught how to kill people by targeting their major arteries.

He denied responsibility for the crime, claiming a workplace injury the year before led to him suffering seizures that left with him "gaps in time in his consciousness" where he did things he could not remember.

He said the only memories he had of events on that day were being at his home with his children and then being in the Joondalup Police Station with his hands covered in blood.

Relationship breakdown turned deadly

The jury was given the option of delivering a verdict that Turner was not of sound mind at the time of the stabbing, but it rejected his defence and found him guilty of murder.

The court was told the couple had been in a relationship for eight years and had two children, but it ended in August 2016 and there were then disagreements about custody arrangements.

Sarah Marie Thomas was the mother of two children she had with Paul Turner before they separated. ( AAP/Supplied )

In submissions on Tuesday, Turner's lawyer Lisa Boston said in the months before the murder her client was depressed and suffering severe headaches as a result of the workplace accident.

Ms Boston said "the tipping point" was when Ms Thomas engaged a lawyer who took the case to the family court and the children were "weaponised" in the breakdown of the relationship.

She described Turner as "a loving and caring father" who was upset at the prospect of having access to his children restricted over the upcoming Christmas period.

The knife Paul Turner used to murder his former partner Sarah Thomas. ( Supplied: Supreme Court of WA )

Mr Mactaggart said the state "utterly rejected" the submission that Ms Thomas's engagement of a lawyer contributed in any way to what happened, saying she, like any citizen, was entitled to get legal advice.

He said the crime was aggravated because it happened in the presence of others, including the court staff member.

The CCTV footage showed Turner to "be cool, calm and collected" and after the murder "sat down with a smug look on his face, saying 'what's anybody going to do about it?'".

'No sentence is enough'

Ms Thomas's family welcomed the fact the matter had been closed, but her father Greg Thomas said it did little to heal the wounds of such a traumatic event.

"I'll never be satisfied, I'll never get my daughter back," he said.

"No sentence is enough."

Her sister, Emma Howden, said their focus was now on Ms Thomas's children and bringing them up in the best way they could.

"We've got some kind of finality now so we can try and move on with our lives and bring up the kids the best we can," she said.

"She was the most gorgeous person you could meet. She had the time for everybody. It's a huge loss in our lives to not have her here."

But they were critical of how such a violent attack could have happened in a courthouse building, which was supposed to be safe.

CCTV captured Paul Turner in a waiting room at the Joondalup court complex before a meeting with his former partner. ( Supplied: WA Supreme Court )

"She should have been safe and she wasn't," Ms Howden said.

"Everyone has that right, whether it be male, female, it doesn't matter, they should be safe and secure walking into a government building.

"I hope it shows the judges are now getting very serious with domestic violence cases and I hope that continues so it stops. It's got to stop."

Turner's sentence was backdated to the day of the murder when he was arrested, meaning he will remain behind bars until at least 2040.