A man who used "commando" knife training to stab to death his former partner and mother of his two children during a mediation session at the Joondalup court complex has been found guilty of murder.

Key points: Sarah Thomas died after being stabbed six times, including three times in the neck

Sarah Thomas died after being stabbed six times, including three times in the neck Her ex-partner attacked her during a mediation session over a financial dispute

Her ex-partner attacked her during a mediation session over a financial dispute Paul Turner said he was having a seizure when he stabbed his former partner

Paul Gary Turner, 43, claimed he was having a seizure when he used a knife to stab 33-year-old Sarah Thomas six times in front of a court officer.

The attack unfolded during a mediation hearing in December 2016 held to resolve differences over money Turner claimed Ms Thomas owed him.

The Supreme Court was told Turner had trained in "commando" knife techniques and knew how to kill people by cutting vital arteries.

The jury was told he used that knowledge to deliberately and intentionally fatally stab Ms Thomas.

Sarah Marie Thomas had two children with Paul Turner. ( AAP/Supplied )

She suffered six stab wounds including three to her neck, one of which severed her carotid artery and was described in court as "non-survivable".

Turner took the knife into the complex either in the pocket of his shorts or in a white file he was carrying.

Turner sat in a waiting room at the courthouse before the meeting where he murdered his ex-partner. ( Supplied: WA Supreme Court )

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.

Attack filmed on security cameras

CCTV footage from inside the court complex was played to the jury during the trial, showing Turner and Ms Thomas going into the mediation room with a court officer.

After a short commotion, Turner could then be observed walking out and sitting on the floor outside the room.

In his evidence, Turner said while "it appeared" he had stabbed Ms Thomas, his actions were not conscious and he had no memory of the events because he was having a seizure.

Sarah Thomas was stabbed six times, with three wounds in her neck that proved fatal. ( Supplied: Supreme Court of WA )

Turner said he suffered a head injury in a workplace accident in January 2015, which left him with a range of health problems including continual headaches, dizziness and seizures.

These left him with "gaps in time" in his consciousness.

Turner maintained he was having one of those seizures on the day of the stabbing and all he remembered was being at home with his children.

The next moment he said he remembered was being in the Joondalup police station with his hands covered in blood.

Police tape surrounds Joondalup Courthouse on the day of the attack. ( ABC News: Briana Shepherd )

The jury was given the option of finding Turner "not guilty by reason of unsoundness of mind".

But it deliberated for just over two hours before convicting him of murder.

Turner will be sentenced in October where prosecutors are set to call for a life jail term with a significant minimum period.

Members of both Turner's and Ms Thomas's families packed the public gallery for the verdict.

Speaking outside court, Ms Thomas's father, Greg Thomas, said the verdict offered little relief.

"There's no winners in this case, just justice for Sarah," he said.