Upon you return to Australia, remove the USA sim card, and re-insert your Australian sim card.
If you're using an iPhone, you shouldn't need to do anything else. If you are having issues, just reset your network settings by going to Settings, General, Reset, Reset Network Settings.
If you're using a Samsung, Android, HTC etc, then you'll need to turn back ON the APN for your Australian network (refer to the set-up instructions you used when you changed the APN settings - this will navigate you back to your phone's settings so you can change back to your Australian APN).