Vic rampage case to hear from third expert

A third psychiatrist is due be cross examined about whether Melbourne's Bourke Street murderer-accused James "Dimitrious" Gargasoulas is fit to stand trial over a rampage that killed six people.

Gargasoulas is facing an investigation hearing before a jury in the Victorian Supreme Court to determine if his schizophrenia means he is unfit to be tried.

Consultant psychiatrist James Walton and Forensicare psychiatrist Douglas Bell have both given evidence about the 28-year-old's mental state.

A third psychiatrist, Andrew Carroll, is scheduled to give evidence on Thursday about his interactions with Gargasoulas.

The accused faces six counts of murder and 28 charges of attempted murder over the January 20, 2017 rampage, as well as other unrelated charges.

Three psychiatrists and a psychologist agree the 28-year-old has schizophrenia.

But the psychologist believes Gargasoulas is fit to stand trial despite his schizophrenic delusions.

In letters read to the jury by defence barrister Theo Alexander, Gargasoulas writes "I was born for a reason" which was to "reveal the truth" and "have the scales removed from your eyes".

In another letter the 28-year-old refers to himself as "King James George Gargasoulas" and "ruler of the universe".