Australia has a number of mobile (cellular) phone companies whose networks span the country and use two different technologies, GSM 900/1800 and CDMA. Roaming from foreign networks may be possible, particularly for GSM.
If you already have a GSM 900/1800 phone but for some reason you need a SIM card, ie your carrier doesn't have a roaming relationship for an Australian carrier, you could puchase a prepaid SIM. These companies will also sell you a phone if you really want to do that.