That was the question the territory's Director of Public Prosecutions posed in the ACT Court of Appeal on Thursday, as he wondered how light jail sentences might become for murderers who had not "mutilated an innocent person" and later laughed about it.

Jayscen Anthony Newby did those things in 2020, knifing his former partner's Tinder date 37 times in front of the Charnwood woman in order to terrify her and assert his dominance.

Yet still, the remorseless 28-year-old killer might serve as little as 10 years behind bars.

Shane Drumgold SC appealed on Thursday against this non-parole period and the 20-year total jail sentence imposed by Chief Justice Helen Murrell, arguing they were "manifestly inadequate".

In doing so, the ACT's top prosecutor described how Newby's jealous rage had built up over several months after the murderer's former partner ended their relationship.

The woman had made it clear to Newby she was no longer interested in being with him after violent incidents that included him slapping her in the face and smashing her phone.

Though he began dating other people, Newby could not handle his former partner doing the same thing. One night, he hid in a bush outside her home and watched as a man left.

Newby's controlling anger reached its peak early one morning in January 2020, when he sneaked into the woman's Charnwood residence and grabbed a knife from her kitchen.

He then went to her bedroom and launched a frenzied attack on Frankie Victor Prineas, a 27-year-old stranger who had only met Newby's former partner a few hours earlier.

"He mutilated an innocent person to terrify his domestic partner," Mr Drumgold told the court on Thursday.

"He didn't know this man. He plunged a knife into him 37 times to send a message to his domestic partner.

"This was an assertion of dominance and entitlement."

Mr Prineas died what Mr Drumgold described as a "relatively slow and painful" death as a result of the attack.

Newby, meanwhile, fled the scene and made a frantic attempt to destroy blood evidence.

About 15 hours after the killing, knowing arrest was inevitable, he surrendered to police.

While on remand, charged with the murder he ultimately admitted committing, Newby was recorded laughing down an Alexander Maconochie Centre phone line when his mother recounted a news article about the case.

As Mr Drumgold pushed on Thursday for a sterner sentence, he emphasised factors like this clear lack of remorse, the number of knife wounds and the background to the killing.

"If that still results in 10 [years as a non-parole period], then where are we going from there when those elements are not present?" he asked.

"Are we looking at single-digit numbers?"

The prosecutor also described the minimum terms imposed on Newby and axe murderer Marcus Rappel, who received a non-parole period of 26 years for killing his former partner, as being "universes apart" despite many similarities between their cases.

Newby's barrister, David Campbell SC, argued the appeal should be dismissed, saying on Thursday there was nothing in Chief Justice Murrell's sentencing reasons that had "infected" his client's sentence with error.

He noted that Mr Drumgold had not taken issue with the judge having nominated 25 years as the starting point for Newby's sentence before giving the killer a 20 per cent discount.

Mr Campbell said giving some lesser discount in response to Mr Drumgold's complaint would "represent tinkering when it is not required".

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He added that Newby's discount reflected the fact the killer "did the right thing" by pleading guilty to the murder charge when there might have been an argument that his former partner had provoked the attack by dating other people when she had been told not to.

In relation to Mr Drumgold's contention that the 10-year non-parole period was not enough, Mr Campbell pointed out that the Sentence Administration Board may well deny Newby his release after that time.

The sentence was still very lengthy, he told the court, for a relatively young man.

A full bench of the Court of Appeal, comprised of Justice Michael Elkaim, Justice David Mossop and Justice Robert Bromwich, has reserved its decision.

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