The Victorian Supreme Court last week heard from the teen's defence he should have been tried separately from his co-accused, and the joint trial in 2020 meant the jury could not 'mentally' put aside the co-accused's statement for a fair ruling, which found the teenager murdered John Bourke on July 15, 2018.

The teens cannot be named due to their age.

Then 15-years-old, the accused and his co-accused, 17, left a party that evening to confront a man who had allegedly sexually assaulted a close friend.

Instead, the pair mistakenly went to Mr Bourke's Derby Road home.

The teen kicked in the front door of the man's home and began assaulting him when he came to the door on crutches.

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Mr Bourke had osteogenesis imperfecta, or 'brittle bone disease', which left him highly susceptible to fractures.

The older boy, then aged 17, punched the 45-year-old twice in the head and stomped on his leg between two and four times.

The 15-year-old continued to attack Mr Bourke even after his older friend tried to stop him, stomping on the man's head twice, killing him.

The teen's defence counsel Rishi Nathwani said last week there had been a "substantial miscarriage of justice" and argued the teen and his co-accused should not have been tried together as it meant the jury had to engage in "mental gymnastics".

Mr Nathwani's arguments hinged on the inability of the jury to follow the judge's directions to ignore the co-accused's interview statement when deciding the teen's guilt.

"I accept that juries are capable of following directions, but there are some cases where the mental gymnastics asked of a jury is such that [separate trials] should occur," Mr Nathwani said.

Mr Nathwani argued the co-accused's interview downplayed his role and amplified the role of the accused, who in an interview the day after the attack said he could not remember much because he was intoxicated.

"What he could remember was two stomps to a person who he believed was someone else, but as it unhappily transpired, it was the deceased. He accepted a flashback where he saw a male with blood on his face and that really was the sum total of it, of what he accepted as far as his involvement in the incident," Mr Nathwani said.

"The blank spaces in the evidence combined with the Crown's case with the interview of [the accused], was filled in and coloured in bright colour by the interview of [the co-accused], and that is where the substantial prejudice arose."

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The co-accused was sentenced to three years in a youth justice centre after he pleaded guilty to recklessly causing serious injury and home invasion.

Senior Crown Prosecutor Christopher Boyce said the jury was able to follow directions given the cause of death and circumstances around the cause of death - the accused's head stomping - were understood.

"[The co-accused] says two things really, he says, look I didn't stomp on his head and I attempted to withdraw, or I withdrew and I attempted to get [the accused] to stop," he said.

"So it's difficult to see how anything ... when the alignment between the [accused] and [co-accused] interviews are understood in terms of who was stomping where, that there's any real prejudice that was contained in [the co-accused's] interview that cruelled the pitch, or ... meant that the jury could not follow her Honour's clear directions."

Ultimately, the appeal judges agreed.

In court documents detailing their decision, the appeal judges noted the trial judge ordered the questions of the jury's deliberations to lessen any unfair prejudice to the teen.

"[The trial judge] gave repeated and emphatic directions to the jury at various stages of the trial that they could not use [the co-accused's] record of interview when considering the prosecution's case against the applicant," the document read.

"She also provided the jury with a 'question trail', which was designed to ensure that the jury should consider the case against the applicant first, and only then turn to the case against [the co-accused]."

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This was the teen's final avenue of appeal in the Supreme Court.

Any further appeals will need to be taken to the High Court.

The appeal judges decision statement reads in its final notes the teen, "has failed to persuade us that the jury might have failed to have understood, or to have applied, the trial judge's frequent forceful directions designed to guard against the jury's possible misuse of [the co-accused's] record of interview in their consideration of the applicant's case".

"Indeed, there is nothing in this case that might persuade us that the applicant has suffered a substantial miscarriage of justice," the documents read.

"Appeal must be refused."

The teen was sentenced to 14 years in jail with a non-parole period of nine years.

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