The now 18-year-old was sentenced in December last year for murdering Richard "Dick" Cater and seriously assaulting two of Mr Cater's friends.

The jail term would be suspended after eight-and-a-half years, meaning he would be released in 2027 after accounting for his pre-sentence custody.

The teenager had pleaded guilty to murder, intentionally inflicting grievous bodily harm and assault occasioning actual bodily harm.

The sentencing judgment by Justice Michael Elkaim states that the attack happened in March 2019 after the teenager took illicit drugs, including cannabis and LSD, he bought on the dark web.

He fell into a drug-induced psychosis and began roaming the streets near his Palmerston home.

Eventually, he came upon a carload of people who were pulling up at Mr Cater's house before he viciously attacked them.

He bit and punched two of the victims and pulled one of their heads forward so hard he fractured her spine.

Mr Cater ran to get a spade to defend his friends, but the teenager overpowered him and viciously stomped on his head as he lay prone in a gutter where he never regained consciousness.

On Monday in the ACT Court of Appeal, the appellant's lawyer, Jon White SC, argued the sentence was manifestly excessive because youth justice principles were ignored, the offender's drug-induced psychosis should have been considered a mitigating factor and the non-suspended portion was treated the same as a non-parole period for adult offenders.

Mr White said the principles included a young offender's personal circumstances their prospects of rehabilitation, which "are front and centre in the sentencing exercise".

He said the offender had a lower moral culpability because of his immaturity at the time of the offending and that his state of development and his past and present family circumstances - including violence and mental health issues - should afford him leniency.

"There was no preplanning in this case. He is a person who is otherwise quite blameless," Mr White said.

"It was a normal school day. The kids went to McDonald's that afternoon. The appellant took LSD then later took another tab. Nothing in those circumstances that indicate this was adult-like offending."

Mr White said it was "a child committing an inexplicable crime completely out of character".

"The failure to do this (considering the youth justice principles) was a significant error," Mr White said.

"The error has manifested itself in an unjust sentence."

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In relation to the drug-induced psychosis, which the sentencing judge found to be neutral, Mr White said it should be mitigating because there was a "very strong and quite unambiguous finding" that the impacts of the drugs were not something the appellant could not have anticipated.

He said jail should be a last resort and if imposed, it should have the "shortest appropriate term".

In response, Katie McCann for the respondent, the Director of Public Prosecutions, said the failures to cite youth justice principles in sentencing remarks did not mean errors had been made.

Ms McCann said the context of the sentencing reasons was important and that the appellant's maturity and development had no significance.

"There was nothing in the material which suggested the appellant was immature or his state of development not reflecting his age," she said.

Ms McCann said the drug-induced psychosis was the critical factor and that there was no dispute about its impact at sentencing.

She said that there were no rules or principles regarding the suspended and non-suspended portions and that they do determine the excessiveness of a sentence.

"This sentence had to reflect the extreme nature of what's happened," Ms McCann said.

"He did have favourable subjective features ... but counter to that was a set of offences that were horrific."

Ms McCann said that while a young offender's rehabilitation was a primary consideration, other factors - including denunciation and the objective seriousness of the crime - "could not be overshadowed here".

She said the sentence was not unreasonable as Justice Elkaim had achieved justice in balancing the extremes of the crimes and a young offender with no criminal history.

The appeal was heard before Chief Justice Helen Murell and Justices David Mossop and Tom Thawley.

The judges have reserved their decisions.

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