There are certain thing Google takes into account when attributing a site to a particular location:
- country-level domain (com.au would be ideal)
- where the website is hosted
- what target is set in Google Webmaster Tools (if any)
- whether there are location-identifying keywords on your site (address, words like "Australia", "in Australia", etc.)
Besides, some of these signals are stronger than others. For example, if your site is hosted in Hong Kong, it is a .COM site and the address on it says "Hong Kong", even if you set your geotarget as the US, Google is not so likely to honor it.
Anyway, you can just look where your website shows up for user in different locations (http://goo.gl/CRbCuw) and get an idea what region of the world Google considers you relevant for.