US iphones working in Australia?

I have found an iphone on ebay that i would like to buy. Its from the US and has been used before. But i don't know if the phone will even work in australia? The seller does not mention if the phone is locked or unlocked. Help
Also if it won't work in Australia is there any way i can make it?

Added (1). by the way its an iphone 4s

Unless the phone is unlocked, it will not work. If it were unlocked, you can bet your bippy that that fact would be in the listing and you'd be paying extra for it. You do not want a phone you have to hack. You're going to have to accept the fact that an iPhone that works for you is going to be pretty expensive because you need factory or carrier unlocked.

Find out if your service is CDMA or GSM. Get an iPhone 4S or newer that uses sim cards. Buy a factory unlocked phone of the right GSM Or CDMA system and you got no problem. Most non US carriers are CDMA and that menas you need a Verizon or Sprint iPhone form the U S. If they don't mention it its not unlocked but iPhone 5 and newer CMDA and probably iPhone 4S CDMA phones use international sim cards.