Fayez Hatahet admitted fighting with the Free Syrian Army against the al-Assad government, but he claimed he initially travelled to the country to free a brother-in-law he believed was being held captive there.
Hatahet pleaded guilty to engaging in hostile activity in a foreign state and was sentenced in December 2022 to a maximum of five years behind bars.
His sentence was due to expire in August 2025, while he was handed a non-parole period of three years.
In an appeal lodged in 2023, Hatahey's lawyers successfully argued the sentencing judge had failed to take into account that it was extremely unlikely he would be granted parole due to the nature of his crimes.
The NSW Court of Criminal Appeal re-sentenced Hatahet to a period of four years in prison that would expire on August 23.
In reviewing the decision, the High Court ruled on Wednesday that the top court in NSW made the wrong judgment and ordered Hatahet to serve the original five-year sentence.
The purpose of presuming that parole would not be granted to those involved in terrorist activity was for the protection of the community, the majority of the High Court judges agreed.
"It would make little, if any, sense to reduce a sentence of imprisonment, which otherwise is of a severity appropriate in all the circumstances, because of that presumption," three judges said in their written decision.
The law requires that a person convicted of certain terrorist-related activities should not be granted parole unless the attorney-general finds there are exceptional circumstances justifying the decision.
Hatahet's parole was refused when the attorney-general found no such circumstances were warranted in his case.
Following his arrest, Hatahet was held in Goulburn jail's notorious High Risk Management Correctional Centre, the conditions of which were accepted by the court to be "extremely onerous".
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