The High Court has ruled a five-year jail term handed to a Sydney man convicted of fighting with terrorist groups linked to Al Qaeda in Syria is not excessive despite Commonwealth law stopping him from being granted parole.

Fayez Hatahet from Punchbowl in Sydney's south-west was arrested in April 2020 after making a number of trips to Syria in 2012 and 2013 to fight alongside Jabhat al-Nusra.

The dual Australian and Jordanian citizen had claimed his initial reason for travelling to Syria was to try to secure the release of his brother-in-law who he believed was being held by an armed group, but he also then joined fighting in the war-torn nation.

Hatahet pleaded guilty and in December 2022 was sentenced to five years' jail, with a three-year non-parole period.

The sentence was backdated to August 2020. He has remained in the Goulburn Supermax prison ever since.

Hatahet's lawyers challenged that sentence in the New South Wales Court of Criminal Appeal, arguing it was excessive given he was not going to be granted parole after three years.

Commonwealth law states people jailed for terrorism offences can only be granted parole by the federal attorney-general in "exceptional circumstances".

His lawyers said the sentencing judge should have taken that into account when calculating his jail term, and handed him less time behind bars as a result.

They also argued the conditions of his detention, in the highest security wing of the Goulburn jail, were "extremely onerous" and his family had "suffered greatly" as a result.

The New South Wales Court of Criminal Appeal agreed and reduced the sentence to four years jail, prompting a subsequent challenge from Commonwealth prosecutors in the High Court.

All five of the High Court justices hearing the case agreed the New South Wales Court of Criminal Appeal made a mistake to insist someone's prospect of early release from jail should be considered when handing down a sentence.

Acting Chief Justice Michelle Gordon, and Justices Simon Steward and Jacqueline Gleeson said, in their published decisions, that Federal Parliament had expressly decided to limit the options for parole for terrorism offenders.

"In short, any such reduction would undo the very work the presumption was intended to do."

The three justices said it would be "practically impossible" for a sentencing judge to predict whether "exceptional circumstances" existed in the future, when a convicted offender was to be considered for parole.

"That judgment is to be contrasted with a court's appreciation of the seriousness of the offending and the conditions of imprisonment known to it at the time of sentencing.

"Such matters properly bear upon the sentencing task."

Justice Jayne Jagot agreed parliament had been clear in its rationale for limiting when convicted terrorism offenders are granted parole.

"The legislative intention was to create a presumption against the release of certain offenders … in order to satisfy 'the overriding need to protect the community,'" she said in her published reasons.

"Subversion of legislative intention of this kind … is impermissible."

Justice Robert Beech-Jones said the New South Wales Court of Criminal Appeal had been wrong to try to recalculate Hatahet's sentence, in adjusting his time behind bars to four years.