Roaming is the word used to describe using your mobile phone on another network for a short period, while still being billed by your existing provider. Your mobile phone number remains the same while roaming. When you are roaming on another network the temporary mobile phone company will bill your usual mobile phone company for calls you make while roaming on their network. (Note: It is important to note that even calls you receive while roaming when you are overseas may be charged to you and not to the caller.)
Here's my advice: DON'T ROAM! Using your Australian mobile phone in Indonesia and switching to roaming can be prohibitively expensive. I have heard of instances where a tourist's roaming bill amounted to more than all his airfares and hotel expenses combined!
What to do?
1) As soon as you arrive in Bali (or wherever), remove your Australian SIM card from your mobile phone. (Keep it in a safe place as you will want to re-insert it as soon you have arrived back in Australia.)
2) Buy a prepaid Indonesian phone card. They come in various denominations and start from as low as Rp.6000 (about 60 cents) and you can always top up later on. The "3" card has the most extensive coverage in Bali. Other providers may offer better coverage in other parts of Indonesia. The "3" card is sold in countless shops all over the island.
3) Insert the new phone card in your mobile phone. Now call your friends and tell them your new mobile number which is written on the wrapper of the card. All incoming calls while in Indonesia are FREE, regardless of where they originate. You pay the local rate for local calls and a very low rate for calls to Australia. Unlike home, your Indonesian SIM card is prepaid so there is no need for a contract. On completion of each call, the provider sends you an SMS which tells you how much credit you have left. You can top up your credit by paying an additional amount of money at any one of the many shops selling the "3" card.
Please note: the shopkeeper who sells you the card will probably tell you but just in case he doesn't, here's how you make an overseas call (e.g. to Australia): dial 01088, followed by the country code for Australia 61, followed by the area code, say 2 for Sydney, followed by the actual phone number.
P.S. Your Indonesia SIM Card requires a SIM-unlocked GSM 1800 compatible international cell phone. If you have a locked compatible GSM phone, you can easily unlock it. To do so, visit www.unlockingcodesforphones.com.