Accused killer James "Dimitrious" Gargasoulas planned to injure at least one person in the Bourke Street rampage to keep police busy so he could be free to contact Aboriginal royalty and promote his belief that he's Jesus Christ.
The 28-year-old is facing an investigation hearing in the Victorian Supreme Court to determine if he is fit to stand trial over the 2017 event in central Melbourne that killed six people, including two children.
Psychiatrist Lester Walton on Wednesday said Gargasoulas is schizophrenic and delusional.
"He has cosmological, Aboriginal and religious beliefs that he is the messiah and it's all mixed up together," he told the jury.
Dr Walton said Gargasoulas believes "he is the second coming of Christ".
"He has said straightforwardly to me that he is the messiah."
The psychiatrist added Gargasoulas had a "chain of irrational logic" about what he was doing during the rampage in the Bourke Street mall on January 20, 2017.
"He was being pursued by police and he wanted to get rid of them in order to be free to pursue the promulgation of these ideas (that he is the messiah)," Dr Walton told the court.
Gargasoulas told Dr Walton he tried to tell the public about what he needed to do while driving in circles near Federation Square before he headed to Bourke St.
"He was actually yelling out things and that was his attempt to start the process of informing the public ... about this important cataclysm that was about to happen," Dr Walton said.
The psychiatrist said Gargasoulas believed injuring someone on Bourke St would stop police from following him and he would be free to "contact Aboriginal royalty".
"Now, I'm not going to put that forward at this stage as a comprehensive explanation, but that's one theme that's emerged," the psychiatrist said.
While being cross examined by prosecutor Andrew Tinney SC, Dr Walton - a witness for the defence - said he had spent about three hours with Gargasoulas.
Mr Tinney said Forensicare psychologist Michael Daffern, a prosecution witness, spent more than seven hours with Gargasoulas.
Professor Daffern believes Gargasoulas is fit to stand trial.
"He concluded that this man remains fit and clearly I was of a different view," Dr Walton said.
He also told the court there is no disagreement about Gargasoulas' schizophrenia diagnosis, but Prof Daffern holds a contrary view about whether Gargasoulas understands the criminal trial process.
"Professor Daffern agrees with all of us in the sense that the diagnosis is schizophrenia, that this man remains unwell, psychotic," Dr Walton said.
Gargasoulas faces six counts of murder and 28 charges of attempted murder over the rampage, as well as other unrelated charges.
The hearing before Justice Lex Lasry will resume on Thursday.