A forensic psychiatrist has stood by his opinion that Mathew Flame didn't have a disease of the mind when he killed his best friend, despite two other experts disagreeing.
Professor David Greenberg said he knew the two psychiatrists - who have prepared reports for the defence - well, saying "they are ex-students of mine".
Earlier on Friday at Flame's NSW Supreme Court trial, Justice Richard Button gave the jurors more legal directions but reassured them it was not "an impossible legal maze".
Prof Greenberg, a crown witness, has testified that in his opinion Flame was in a drug-induced psychosis at the time he fatally bashed Liam Anderson, believing he was a demon about to kill him.
The 22-year-old apprentice plumber has pleaded not guilty by reason of mental impairment to murdering Liam Anderson, son of Angry Anderson, singer with hard rock band Rose Tattoo.
He was seen repeatedly stomping on his mate's head shortly after sunrise on November 4, 2018 in a Queenscliff park on Sydney's northern beaches.
Prof Greenberg has testified that in his opinion Flame did not have a disease of the mind when he attacked his friend
While he believes Flame now qualifies for a diagnosis of schizophrenia, he said such a disorder developed more than seven weeks after he was taken into custody.
Under cross-examination from John Stratton SC, he said he was aware the two defence psychiatrists expressed the view Flame was suffering from schizophrenia at the time of the killing.
But Prof Greenberg said while he agreed Flame didn't know he was doing wrong and could not reason about whether his conduct was wrong at the time, he did not then have a disease of the mind.
His drug psychosis had been a very brief episode of several hours duration.
But a diagnosis for schizophrenia could involve a first episode of at least one month followed by signs of illness for six months, Prof Greenberg said.
He disagreed with a suggestion that most people who commit homicide and have a mental condition at the time, were never previously diagnosed with the condition.
He told prosecutor Gareth Christofi that factors including Flame's trauma over killing his friend, his incarceration and his use of illicit drugs in jail could have been related causally to his subsequent development of schizophrenia.
The trial continues.