The woman has been charged with the “kidnapping, arbitrary detention and abduction” of the man, named as 31-year-old Florent G, her defence counsel Simon Despierre told AFP.
Relatives of the missing man from the western city of Nantes had requested the case be kept out of the public eye after the woman was arrested on November 28 at Paris's Charles de Gaulle airport, the source said.
Despierre said the 59-year-old was formally placed in detention two days later after questioning.
The circumstances of the case remain unclear.
Florent G was last seen at a hotel in Andorra and his family reported him missing after he did not return to France from the small landlocked country in the Pyrenees mountains, the source said.
He was said to be a proponent of “woofing”, a practice that allows tourists to work on a farm in exchange for room and board.
There is no indication of how he came into contact with the Australian woman, reportedly a doctoral student in architecture who stayed with him in the hotel, according to the Nantes daily Presse Ocean.
“Later, she would say she saw the door of their room open and traces of blood inside and that she did not know what could have happened to her companion,” Presse Ocean reported.
“She told the examining magistrate she did not harm Florent G,” Presse Ocean added.
The Nantes public prosecutor declined to comment on the ongoing case.