David Hicks - a 29-year-old convert to Islam - was arrested in Afghanistan in late 2001 and was among the first of the Guantanamo detainees to be charged.
Hicks' US civilian lawyer said the Australian lawyer, Stephen Kenny, was no longer needed.
"This is something that's been in the works for a while, just differing needs that we have in the case with respect to its development over time," Joshua Dratel said on Wednesday.
Denying media reports to the contrary, Dratel said Kenny had not been sacked because his approach to the Australian government in pushing for the release of Hicks from US custody had been too confrontational.
"It's not true. Obviously, our position remains the same, which is that the Australian government should be pressing as hard as possible for David's release," Dratel said.
Hicks has pleaded not guilty to aiding the enemy, attempted murder and conspiracy to commit war crimes.