Jury to decide if Bourke Street accused is fit to stand trial

A driver accused of killing six people on Melbourne's Bourke Street is on his sixth antipsychotic medication, including a dangerous "last resort" treatment, the Supreme Court has heard.

James "Dimitrious" Gargasoulas this morning faced day one of a hearing before a jury to determine if he is fit to stand trial on six counts of murder and 28 counts of attempted murder over the January 2017 incident.

The court heard Gargasoulas had treatment-resistant paranoid schizophrenia with "Messiah delusions" and was in early stages of treatment with a potentially life-threatening drug.

Both the defence and prosecution agree Gargasoulas has schizophrenia and suffers delusions, but prosecutors argue he is "not consumed by his delusional beliefs" and "capable of redirection" away from them.

Forensic psychiatrist Lester Walton told court he spent about four hours assessing Gargasoulas in custody.

He said his condition, where "paranoid delusions predominate", had become chronic and he had not responded to treatment.

"It's set and it's enduring," Dr Walton said.

Dr Walton said he believed Gargasoulas was unable to enter a plea due to his delusions, believing "he was acting under divine instruction" when he allegedly drove through pedestrians.

"'My mission is to reinstate God's law, which will redeem myself,'" he quoted from Gargasoulas.

"I will become king before the end of this court case.

"If I don't reinstate God's law, we are all going to die."

Dr Walton said Gargasoulas could not "meaningfully enter a plea", regarding that as "irrelevant" to his mission.

"He wants to be recognised as the king. This will happen, apparently, during the course of the trial once a comet arrives," he said.

He added Gargasoulas believed that he needed to "have his day in court" to persuade everyone of his world view, "or we will all perish".

Defence barrister Theo Alexander argued Gargasoulas was unable to meet three criteria required of an accused person facing trial, including his ability to enter a plea and exercise the right to challenge jurors.

He also argued Gargasoulas was unable to understand the substantial effect of evidence or instruct his lawyer.

"What's not in dispute in this case is that Mr Gargasoulas has a mental illness," Dr Alexander told the jury.

“What is in dispute seems to be the exact nature and impact of his mental illness and the effect on his ability to understand and participate in the criminal process.

"He's very unwell to the extent we should have great concerns about his ability to stand trial."

Several other experts are expected to testify during the fitness hearing, including forensic psychiatrist Andrew Carroll and forensic psychologist Michael Daffern.

The hearing continues.