If you enable roaming it will however that is an expensive way of doing it. Purchasing a travel pack before you go is the best option.
Can be very expensive:
• Make and receive a call:$1.50 / min
• Send SMS:$0.50 / SMS• Receive SMS:Free•
$1.00 / MB
Be especially mindful that phones these days do a lot of syncing of data behind the scenes (for example if you take movies or photos these might auto sync and use a lot of data.
One tip if you use google maps is you can save copies of maps on your phone (google it) so your maps (and directions) works when offline and use no data downloing info.
Travel packs are a solution but unfortunately offer no protection to cost over runs (phone/sms is not an issue) but once you use up the small data allowance then you're back on expensive data. If you have an Android phone you should be able to set local (phone based) warnings and cut offs to your data usage. (google it)
Personally I would always suggest just picking up a SIM card when you land at the airport. This will likely have lots of data and free local calls etc. and be quite cheap. Being prepaid you can't be stung. You can use a data app like WHATSAPP to stay in touch with a few important people back home and send snaps etc. via the app.
Only reason to keep the Aussie SIM is if you must get contact from unknown people back home while overseas.
Have a good trip.