Jayscen Anthony Newby's 20-year prison term was set aside on Thursday by the ACT Court of Appeal, which replaced it with a 24-year stretch after finding it to be inadequate.

Three judges also scrapped his 10-year non-parole period, describing it as "plainly unreasonable and unjust", and ordered the Charnwood man to instead serve a 15-year minimum term.

Newby appeared in court via audio-visual link from Canberra's jail as one of the appeal judges, Justice David Mossop, pronounced the new sentence.

The 28-year-old has been behind bars since his arrest in January 2020, about 15 hours after he crept into his former partner's suburban Belconnen home and grabbed a knife from her kitchen.

Newby took the weapon to the woman's bedroom and switched on the light, startling her while she was in bed with a man she had met on the dating app Tinder.

Without saying a word, he began to stab and slash at this innocent stranger, Frankie Prineas, inflicting 37 knife wounds before he fled and left the profusely bleeding victim slumped against a wall.

Mr Prineas, 27, went into cardiac arrest and lost consciousness as paramedics raced him to hospital, where attempts to revive him failed.

Newby, who surrendered himself to police the same day, later pleaded guilty to a murder charge.

In June last year, former ACT chief justice Helen Murrell sentenced the killer to 20 years behind bars and ordered him to serve at least half of that term without parole.

The length of the sentence left Mr Prineas' father, Victor Prineas, "absolutely stunned" and in what he described as "total shock".

ACT Director of Public Prosecutions Shane Drumgold SC later challenged the severity of the jail term on four grounds, which included the total sentence and non-parole period being "manifestly inadequate".

"[Newby] mutilated an innocent person to terrify his [former] partner," Mr Drumgold told a Court of Appeal hearing in February.

"He didn't know this man. He plunged a knife into him 37 times to send a message to his [former] partner. This was an assertion of dominance and entitlement."

The territory's top prosecutor also wondered how light non-parole periods might become for other killers, highlighting factors like the number of knife wounds in this case and Newby's evident lack of remorse.

Mr Drumgold noted that, while Newby was on remand, the killer was recorded laughing down a prison phone line as his mother read out what he thought was a "hilarious" news article about the murder.

"If that still results in 10 [years], then where are we going from there when those elements are not present?" Mr Drumgold asked. "Are we looking at single-digit numbers?"

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On Thursday, the court upheld Mr Drumgold's inadequacy argument and two of the other appeal grounds.

"The result [was] plainly unreasonable and unjust given the circumstances of the offending," Justice Mossop, Justice Michael Elkaim and Justice Robert Bromwich said.

The three judges accordingly resentenced Newby, lengthening the overall jail term and non-parole period on account of factors like the 28-year-old's lack of remorse.

They also noted that limited prospects for rehabilitation had been identified.

With the sentence backdated to start when Newby was arrested, he will not be eligible for parole until January 2035. By that time, he will be 41.

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