The prosecutor, Air Force Col. Morris Davis, had said earlier he would ask for a sentence of about 20 years, on par with the punishment for American Taliban fighter American John Walker Lindh.
"We will argue for something substantially less than John Walker Lindh," Davis told reporters on Thursday, without elaborating.
Hicks, a 31-year-old Muslim convert accused of joining al Qaeda during the U.S.-led invasion of Afghanistan, pleaded guilty Monday to providing material support for terrorism. Under a prisoner-exchange agreement between the U.S. and Australia, he will serve his sentence in his home country.
Hicks is scheduled to appear before a military judge Friday to enter his guilty plea under oath. His attorneys had said their client was considering a plea deal to end his five-year imprisonment at Guantanamo. Davis dismissed speculation that the guilty plea was insincere.
Hicks, a 31-year-old Muslim convert, allegedly attended terrorist training camps in Afghanistan in early 2001 and reported to an al Qaeda commander after the Sept. 11 attacks. He is not accused of fighting against U.S. or coalition forces.
Davis said Hicks must be held accountable.
"It's not just the big strategic thinkers that have carried out the deaths of thousands of people around the world," he said. "It's also those tactical soldiers out on the front line that need to be held accountable."
Defense attorneys did not appear at the press conference.
The former kangaroo skinner is the first detainee to appear before reconstituted tribunals. The U.S. Congress approved the new system to prosecute Guantanamo detainees last year after the Supreme Court struck down as unconstitutional the Pentagon's previous efforts to try them. Prosecutors say they expect to charge as many as 80 of the 385 men held at Guantanamo.
Human rights groups say the tribunals are illegal because they do not offer the same protections as U.S. courts, but the military insists they are fair and appropriate to try terror suspects.