A recent ACT Supreme Court appeal judgment lists cases dating back to 2010 in which non-parole periods have been given to offenders sentenced for crimes committed in lawful custody.

It has been normal for courts to essentially top up existing jail terms when offences are committed behind bars, and to reset non-parole periods which are automatically cancelled by the new sentences.

But, as Justice Chrissa Loukas-Karlsson observed, this is not allowed, with the ACT's Sentencing Act precluding the setting of a non-parole period for such offences.

When followed, this means people sentenced to jail time for offences committed in custody must serve their terms without any chance of early release.

Cases in which this provision has been overlooked include two where axe murderer Marcus Rappel has been sentenced to more time behind bars, and had his non-parole period reset, for offences he perpetrated in Canberra's Alexander Maconochie Centre.

Another relates to Cedric Roberts, a notoriously violent prisoner sentenced last year for tipping boiling water mixed with jam over three fellow inmates and stabbing a fourth in the mouth with a shiv.

Justice John Burns adjusted Roberts' non-parole period at the time despite these offences having been committed in custody, making him eligible to apply for release in April this year.

Roberts has now sought parole, but the ACT Magistrates Court heard on Friday afternoon that a decision on his application had been put on hold until the conclusion of two ongoing court cases.

The 23-year-old was supposed to be appearing before magistrate James Stewart for sentence in relation to one of the outstanding sets of proceedings.

This case included multiple assaults and incidents of property damage, all of which occurred in Canberra's jail.

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But Mr Stewart said he was "deeply concerned" about going ahead because he believed Roberts would be disentitled to parole if he did in light of the recent judgment.

He said he thought it would be repugnant, or "something akin to repugnant", for him to give a young Aboriginal man a hefty head sentence with no possibility of parole when Roberts had already been eligible for it for six months.

"It's horrible and it's not right," the magistrate said.

Mr Stewart asked prosecutor Juanita Zankin and Roberts' lawyer, Darryl Perkins, to consider whether the matters might be more appropriately dealt with in the Supreme Court and how to get them there.

Roberts, who is currently in the Goulburn Correctional Centre despite being in custody for ACT offences, put on a green cap midway through his audio-visual link appearance in court.

He asked Mr Stewart why he could not have just been sentenced years ago when he committed the crimes over which he was still waiting to learn his fate.

The magistrate replied that, in some respects, he wished that had happened.

"If I sentence you [now], you have to serve your whole head sentence and you can't get parole," Mr Stewart explained.

"That's why it's no good for you [to proceed]."

The case was listed for mention on October 22, when the next steps may become more clear.

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