Sinan,
I am sorry I can't be of much help.
I used to study in Australia and I remember that I brought a mobile phone with me from Norway which worked quite well down there. I remember buying a Vodafone SIM for my Ericsson. It was a prepaid system which I used just so my wife could reach me if needed. Later I bought a phone when the battery of my old Ericsson died. I can't remember that the SIM's were particular expensive, and I filled it for about $20 AUD a month. If you are going to call international numbers, I have no idea of what the hourly rate is. I used to call from my flat phone to Norway using a SuperSaver card of some sort (you called using (free) local line and the international call was deducted from the prepaid card).
Maybe browsing a couple of phone-companies would give you an idea?


Link: http://www.cellular-news.com/coverage/australia.php


The AUD was about half the worth of USD when I was down under…