David Hicks is set to appeal his US conviction for supporting terrorism following a court decision in the United States that quashed a similar charge.
The move comes after Salim Hamdan, a former driver for al-Qaeda leader Osama bin Laden who served a prison term for material support for terrorism, had his conviction thrown out after the US appeals court ruled the charge was unlawfully applied retrospectively.
Mr Hicks pleaded guilty to the same charge under a deal allowing his transfer to Australia in 2007 from a US military jail at Guantanamo Bay in Cuba after five years in detention.
Stephen Kenny, the Australian lawyer for Mr Hicks, said the decision by the US appeals court caught them by surprise on Wednesday, but a challenge to his conviction was already on the cards.
'It gives us great hope and David is very pleased to hear about the decision and so was I,' Mr Kenny told AAP on Wednesday.
'It confirms to me what I believe was that David was never guilty of any offence and the offence he was eventually forced to plead to didn't exist in international, Australian or US law at the time when he was there.
'It was created and made retrospective.'
Mr Hicks said on Wednesday he always believed the conviction, which allowed him to complete his sentence in an Adelaide's Yatala Prison, was 'doubtful' and should be thrown out.
'President Obama himself acknowledged the unfairness of the system and that is why he replaced the 2006 Military Commissions Act,' he 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.'
Mr Hicks' former lawyer Dan Mori said the US case could help Mr Hicks get closure on his incarceration and eventually clear his name.
'It would be great for some official recognition that what he was put through was not fair and not just,' he told ABC television.
Mr Mori pointed out the retrospective defence was one of the first motions he filed on behalf of Mr Hicks in 2004, but it was 'scoffed at'.
Prime Minister Julia Gillard told reporters in New Delhi Mr Hicks was convicted under US, not Australian, law and what he did in light of the US case was up to him.
A spokesperson for Attorney-General Nicola Roxon said Mr Hicks would no doubt want to review his case, but added the prospects were not immediately clear.
'He was not a party to the (Hamdan) case, and there are potential appeal proceedings,' the spokesperson said.
The government also could not comment on what the Howard coalition government knew at the time of Mr Hicks' guilty plea before the 2007 federal election.
'The government is examining the case to consider any local implications,' the spokesperson said.
The chief prosecutor in the Guantanamo Bay military commissions in 2008, retired Colonel Morris Davis, said the Hamdan decision was a 'body blow' for the US government.
'I personally approved the material support charges against Hamdan and Hicks in February of 2007,' he wrote in an article on the US website Mother Jones.
'I realised later on that I was mistaken on both counts.
'I hope this puts an end to the US making up ex post facto war crimes.'
Opposition Leader Tony Abbott told reporters in Sydney the matter was one for the US justice system and 'let's see how it goes'.
Australian Greens legal affairs spokeswoman Penny Wright called for an independent inquiry into Mr Hicks' detention and what the Howard government knew about his treatment at Guantanamo Bay.
Mr Hicks was captured in Afghanistan after the September 11 terrorist attacks on the United States.