The solicitor for David Hicks says Freedom of Information (FoI) documents reveal the Howard government knew the former Guantanamo Bay detainee would not get a fair trial.

Papers from the Department of Foreign Affairs and Trade (DFAT) and the Attorney-General's Department were obtained by human rights advocate and lecturer Aloysia Brooks, who is married to Mr Hicks.

They include emails, advice and cables between officials in Australia and the United States.

Mr Hicks is trying to get his conviction for providing material support for terrorism overturned on appeal.

In 2012, a US appeals court in Washington ruled in the case of another Guantanamo convict, Osama bin Laden's one-time driver and bodyguard, Salim Hamdan, that providing material support for terrorism was not recognised as a war crime during the time Mr Hamdan and Mr Hicks were in Afghanistan.

They were prosecuted under a US law enacted in 2006 and the court said it could not be applied retroactively. The ruling overturned Mr Hamdan's conviction.

Mr Hicks pleaded guilty to material support for terrorism in March 2007, but his US lawyer, Wells Dixon, says he did so under duress.

Mr Hicks' solicitor Stephen Kenny says the FoI documents add further weight to his case.

"The government actually knew that David Hicks wouldn't get a fair trial and that he had been mistreated and that their statements, especially in 2004 and 2006, were inconsistent with what they'd actually said," he said.

"They were misleading the public.

"It means that his appeal is now more important because it confirms what we knew and that was that the system was unfair and consequently, he should never have been forced to plead guilty to a crime that doesn't exist."

The ABC has contacted the Attorney-General's Department for comment.