The father of David Hicks, the first terror suspect to plead guilty before a court at Guantanamo Bay, says his son did it so he could return to Australia.
David Hicks has been held at Guantanamo for five years
Terry Hicks said the plea - to a charge of providing material support for terrorism - was "a way to get home".
The US and Australia had agreed that the 31-year-old Muslim convert could serve out his sentence in his homeland.
Lawyers for David Hicks, meanwhile, met prosecutors on Tuesday to finalise details of his plea bargain.
Hicks pleaded guilty to the charge on Monday at the opening hearing into his case at the detention camp in Cuba.
The hearing was the first under a new military tribunal system introduced for the detainees, a system which has been condemned by human rights groups.
The Australian, who has been in the camp for five years, was accused of attending al-Qaeda training camps and fighting with the Taleban.
Before the hearing began, lawyers for Hicks had suggested that he did not believe he would get a fair trial and that he was considering a plea bargain to expedite his return home.
His father said that was what had happened.
"It's a way to get home, and that's what he's told us," Terry Hicks told Radio Australia.
"He's had five years of absolute hell, and I think anyone in that position, if they were offered anything, they would possibly take it."
But the chief prosecutor, Colonel Morris Davis, told ABC radio that Hicks would have to set out his guilt in a further hearing.
"He'll have to acknowledge that he's in fact guilty and that he's pleading guilty voluntarily and for no other reason," he said.
This will not take place until both sides agree on the details of the plea bargain, including the sentence.
Col Davis told ABC that prosecutors could seek 20 years, but that time Hicks had already spent in detention would be subtracted from this. He did not yet have a precise number, he said.
Australian Prime Minister John Howard said he welcomed the progress towards the resolution of Hicks' case, although he expressed concern over the amount of time it had taken.
But the Center for Constitutional Rights, which represents many of the detainees, condemned the proceedings.
"Hick's guilty plea should not be seen as legitimising in any way an utterly illegal system of off-shore penal colonies, abuse, and trials that violate fundamental due process rights," it said in a statement.
The US has said it plans to use the new tribunal system to prosecute about 80 of 385 prisoners remaining at the camp.