score:5
The short answer is that hostels exist primarily where people who stay in hostels would want to stay. Proximity is only one consideration; for example, a location adjacent to a rapid transit line but a distance away from the center might be more desirable than one which is closer as the crow flies but inconvenient without a car. As to the Wikivoyage commentary, the fact that hostels are scarce in one area or another does not mean that they are nonexistent.
A quick-and-dirty proxy for hostel availability is to use the maps on the booking site of your choice. While not every property is listed, of course, we are only looking for a rough approximation of where such properties could be found, and in a rich country like Australia I would think the correlation to be adequate for such purposes. If you click down to the state level and use the "show map" feature, all the communities with at least one property listed will show.
Upvote:2
In addition to choster's answer, it should be noted that Working Holiday visa holders can get a 1-year extension to their visa if they work in certain jobs (mainly agricultural ones such as fruit-picking) in a regional area for 3 months. Therefore you will also find youth hostels in major agricultural centres such as Mildura, shown in top left-hand corner of the map included in choster's answer.