I have an overseas GSM mobile phone and have bought a SIM card in Australia – why doesn my mobile phone work?
It may be that your mobile phone is still under contract (SIM-locked) to your overseas mobile phone service carrier and is unable to be connected to another carrier’s network. The ACMA recommends you contact your original overseas mobile carrier to arrange to have the mobile phone unlocked from its original SIM card.
As long as your phone is not SIM- or network-locked, then yes, you can swap yourAustralian sim card in it.. If not you have to get your handset unlocked from your SP or any commercial vendors online like Onlinegsmunlock.com .Oncen unlocked you can use any carrier to it..