No F-35s around in Australia as yet, was only 2 down here for the Avalon Airshow and they came in through Amberley. Williamtown is currently hosting Exercise Diamond Shield however where RAAF F/A-18s have been flying exercises with some USAF F-16s which would presumably be what you've been hearing? Some more details for you here:
http://adbr.com.au/air-warfare-centr...iamond-shield/