Seems to me they would be much better off keeping him themselves and reporting any intel they got to the French.
Keeping one terrorist can't cost that much compared to the GDP of Australia. If the French decide to let him go after the next islamic threat or bombing, he will be back to Oz with better plans in hand.
He was sent back to France because ASIO (Australia's little CIA) screwed up.
Extract of interview (Sunday Program 8 February 2004) regarding this matter with Australia's Attorney General, Philip Ruddock, below.
SARAH FERGUSON: After Brigitte's arrest, Hamsa was put under surveillance and observed while he dumped the print-outs of the military bases in a rubbish bin near his home. He also called Sajid to tell him Brigitte had been arrested. Brigitte was handed over to the Department of Immigration and moved here to the Villawood Detention Centre in Sydney. To the surprise of French authorities, he was deported to France and not interrogated under ASIO's new powers. It would have been the first use of those powers and the Attorney-General was uncertain whether they applied to Brigitte.
PHILIP RUDDOCK: You have to make a judgment in relation to these matters and the security agency made a judgment. One, that it wasn't the best and most appropriate case to test for the first time the powers that were implemented in the revised ASIO arrangements, because it was questionable.
SARAH FERGUSON: But he did meet that test of someone who is going to be able to offer reasonable assistance in regards to a possible terrorist attack?
PHILIP RUDDOCK: Yes. Yes.
SARAH FERGUSON: So why not take the opportunity to use those powers to learn more about his contacts here?
PHILIP RUDDOCK: I said the advice I have is that it was questionable.