Hicks converted to Islam in 1999, trained with the Kosovo Liberation Army during the war against Serbia, and later in Pakistan with Lashkar-e-Taiba, an Islamic terror group fighting to eject India from Kashmir.
In 2001 he moved to Afghanistan, where he met Osama bin Laden, the late founder of al-Qaeda, and was posted to Kandahar to fight against the Northern Alliance. The US alleged he intended to fight against Coalition forces. Hicks denied this.
Hicks was imprisoned for more than six years before reaching a plea deal in 2007, under which he was convicted and returned to Australia.
Hicks, who renounced Islam in Guantanamo, is now challenging his conviction in the US Court of Military Commission after a Court of Appeals ruling last year that the charge could not be applied retrospectively.
His lawyers also said in court documents that Hicks' guilty plea had been forced by his extended detention, torture and abuse.
"Over the course of more than five years, Mr Hicks was repeatedly beaten, sexually assaulted, threatened with deadly violence, injected with unknown substances and subjected to an entire arsenal of psychological gambits ... that had as their aim the destruction of his personality," the documents said.
"He was stripped naked, deprived of sleep for extended periods, cast into solitary confinement, contorted into shapes that no human body should be forced to assume, and told that he would never again set foot on his native soil."