Question concerning the use of a US motorola droid in australia

[A-bo]

A few months ago I traveled to New Zealand and Australia. I brought my Droid with me because i was using it up until the day I left the States. I automatically assumed it would not work down under because it is not a world phone. Therefore, I turned it off for the majority of the trip, thinking that I would use it again when I got back to the States. HOWEVER, I just so happened to turn it on during my last day in New Zealand, and it I was receiving Texts and VM's through the Vodafone signal. Can someone please explain how this works. I am going to back to Australia for a few months, and I was wondering, rathering than getting a pre-paid phone down there, can I simply use my droid on the vodafone signal. Would that be expensive. I am lost when it comes to this