Magento Australia Locale HOWTO
Update: Please note that en_AU is now included by default in Magento so all this is unnecessary.
Simple tip here on how to enable the "English (Australia)" locale in Magento: simply go to app/etc/config.xml and, under config->global->locale->allow->codes add in the text
<en_AU/>. Your config file should have something similar to the following:
You can even do this before installation to allow the selection of the Australian locale during the install process.