David McLeod, from Adelaide in South Australia, said he would leave Australia on Sunday to visit Mr Hicks and his American legal team.
"I'm pleased that the secretary of the Attorney-General's Department has changed his view on the matter," Mr McLeod told reporters.
The visit will allow Mr McLeod and Mr Hicks's American lawyers to meet with their client together ahead of some key court dates in the US and the UK.
The 30-year-old Muslim convert from Adelaide has been held since January 2002 in US military detention at Guantanamo Bay in Cuba, charged with conspiracy to commit war crimes, attempted murder and aiding and abetting the enemy.
He was captured with Taliban forces in Afghanistan in December 2001.