Just had a look at Airservices charges:
http://www.airservicesaustralia.com/...cesJul2013.pdf
Section 3 about terminal charges makes no mention of VFR overflight charges and 3.3 actually says fees at all the major airports are not imposed on gliders, balloons or ultralights.
Section 5 on enroute charges only mentions IFR.
I would be checking all charges for VFR flight if you did not land.