Hicks was returned to Australia in April 2007 following a plea deal that saw him serve a nine-month sentence at home for providing material support for terrorism after nearly six years in Guantanamo.
But his conviction was cast into doubt after the US Court of Appeals ruled that material support for terrorism was not a war crime, and could not in any case be applied to Hicks because the relevant law was passed only in 2006.
Hicks was captured in Afghanistan following the 9/11 attacks in the US and only in 2007 was he charged with providing material support for terrorism.
The ruling was made as the Washington appeals court threw out the conviction of Osama bin Laden’s former driver Salim Hamdan — also once jailed at Guantanamo — on the same charge.
“We have always said the conviction was doubtful and shouldn’t stand,” Hicks told Fairfax Media.
“I want a full investigation. The Australian government knew for years that the system was not fair, but it put me up before it anyway.”
Dan Mori, who represented Hicks at his Guantanamo Bay hearings, said the court decision meant his conviction was unlawful, although the US government is reviewing the ruling and could still appeal.
“It (the charge of material support for terrorism) is null and void for conduct prior to 2006,” he told ABC television.
“The foundation is rotten and the house is starting to crumble.”
Mori said it was now up to Hicks’s current lawyers to decide if they would formally seek to have his conviction overturned.
“I think it would be great for some official recognition that what he was put through was not fair and was not just,” he said.
Steven Glass, who currently represents Hicks, said he was examining the ruling and its implications for his imprisonment in Australia.
“He was detained in South Australia… after he returned, and of course if he was never convicted, there could be an argument that that detention was unlawful,” he said.
Prime Minister Julia Gillard refused to be drawn on the case.
“It’s important to remember Mr Hicks was convicted under US law and not Australian law,” she said in New Delhi, where she is on a state visit.
“What Mr Hicks does in light of that (US) decision is a matter for him and whether or not that case is further appealed by US authorities is a matter for the US.”
In his controversial memoir, Hicks describes “six years of hell” in Guantanamo, where he claimed he endured deprivation and witnessed acts of brutality.
He details paramilitary training in Afghanistan and Pakistan and involvement in conflicts in Kosovo and Kashmir, but claims he was a “political scapegoat” and said he never had extremist intentions.