Regardless of the gadget or software, the default option when "English (Australia)" isn't available is "English (UK)" or similar. And that's usually good enough... but not always. Such is the case with Google's Nexus 5, which leaves local users who are particular about their language settings in the lurch. Fortunately, there is something you can do about it.
Ausdroid's Allan O'Rourke has written a great tutorial on getting your new Nexus 5 to "speak" Australian and it's not difficult at all (though this is variable depending on whether your phone is rooted or not). It involves installing MoreLocale 2 from Google Play and a couple of typed commands, after which you should be able to select "English (Australia)" from the Language & input menu.
Head over to Ausdroid for the full guide, as well as a more detailed explanation of what's required for rooted and non-rooted devices.
Make your Nexus 5 speak Australian [Ausdroid]