Former Guantanamo Bay detainee David Hicks is set to appeal against his conviction for supporting terrorism.

Hicks spent time in the US military prison after being captured in Afghanistan in 2001. He eventually pleaded guilty to providing material support for terrorism.

He was transferred to an Adelaide jail in April 2007 where he served nine more months before his release.

Mr Hicks's lawyer, Stephen Kenny, says he expects to launch the appeal against the charges within weeks.

Mr Kenny says Hicks was convicted for a crime that does not exist in any law, with another case in the United States confirming that fact.

"[David] feels he was wrongly convicted and I think there's a lot of people that believe that that is the case, including me," he said.

"[The] crime he was charged with is not actually a crime ... it was simply invented in Guantanamo Bay effectively for the purposes of the military commission there.

"David has effectively been convicted of a crime that doesn't exist. The crime of aiding the enemy was never a crime in Australian, American or international law, and there's now been a further court case in the United States that has confirmed this, and so we are appealing."

After his release, Hicks said the guilty plea that led to his conviction at Guantanamo Bay was obtained under duress.

Last year, the Department of Public Prosecutions (DPP) dropped a case arguing that the profits of Hicks's memoir should be deemed the proceeds of crime.

Hicks had argued there was no evidence that he had committed any crime.

The DPP did not say why it dropped the case.