If you take an Australian sim with you, you will have to pay the Australia sim roaming rates. For Boost the price are the same as Telstra prepaid roaming, and you need to have "add on" credit purchased to actually use it. It is NOT cheap to roam. Best bet is to get a prepaid sim with some credit from the country you are visiting. Much cheaper rates.
* I am a Boost Australia customer. This is a peer to peer community where users help each other.