In a significant ruling, a US military appeals court has overturned the conviction of an Australian man, David Hicks, who was held in Guantanamo Bay and found guilty years ago of material support for a terrorist organization.

Hicks, arrested in Afghanistan in 2001 and accused of fighting alongside the Taliban against US-led forces post-9/11, was among the prison's first detainees. He pleaded guilty in 2007 to providing material support to Al-Qaeda.

The three-judge panel unanimously struck down Hicks' seven-year sentence, citing a recent civilian court decision that material support for terrorism was not a war crime triable by military courts.

Myles Caggins, a spokesman for military courts, confirmed the government would not appeal the ruling.

Hicks, transferred to Australia in 2007, served the remainder of his sentence there and was released later that year.

Upon learning of the decision, Hicks expressed relief, according to his lawyer Wells Dixon, who argued the appeal. Dixon criticized the military commissions, used at Guantanamo, as an inadequate system of justice.

The case highlights ongoing challenges with Guantanamo's military commission system. Of six people sentenced by these courts since 2006, three have had their sentences reversed, and three more are appealing.

Human rights advocates, including Daphne Eviatar of Human Rights First, emphasize the superiority of the US federal court system for trying terrorism cases, casting further doubt on the effectiveness of Guantanamo's procedures.