It's cheaper to buy a handset in the UK than in Australia for some reason, like you can get a better handset for less money I reckon, but make sure you get it either unlocked or with a network that has an Australian equivalent so you can buy a sim card in Oz. It's cheaper to get pay as you go (or 'prepaid') in Australia than contract as well. Vodafone is probably a good option if you want to buy the handset in the UK because they have cheap sim deals in Australia and you can get a decent handset. Australia doesn't have T-Mobile, but does have 3 and I'm pretty sure it has Orange as well (but no-one I know uses them).