The jury deliberating on the mental fitness of accused Bourke Street killer James Gargasoulas for trial has been dismissed after failing to come to a unanimous verdict.

The jury of eight men and four women had been trying to reach agreement on Mr Gargasoulas’ mental state since Monday afternoon.

He faces six charges of murder and 28 of attempted murder following the deadly rampage in the Bourke Street mall in January 2017.

Experts who gave evidence at the hearing were divided on the whether Mr Gargasoulas, 28, was mentally well enough to face a trial.

Justice Lex Lasry dismissed the jury on Friday morning saying “it is unfortunate we have got to this stage as it means the matter has to run again”.

“There are those among you who had strongly held views and you were right not to compromise.

“These things happen.”

Justice Lasry said the fitness investigation would have to be repeated but suggested it could be unsuccessful a second time.

“Having heard all the evidence, firstly I am not surprised the jury struggled to reach a decision,” he said.

Medical experts who testified in the case were divided over the question of Mr Gargasoulas’s fitness.

Two psychiatrists believe he is unfit for trial but one psychologist disagrees.

The defence argued Mr Gargasoulas was “profoundly psychotic”, believing he is the “Messiah” sent to save the world from a comet, and should not stand trial.

But prosecutors said he should stand trial, arguing he understands his case and the trial process.

The ABC is reporting that Mr Gargasoulas is to be treated at the secure mental health facility, the Thomas Embling Hospital.

Friday is Justice Lasry’s last day as a Supreme Court judge, as he must retire because of his age.

The Gargasoulas case will be handed to Justice Mark Weinberg. The matter is set to return to court in late July.

-with AAP