It's partly a result of immigration history. IIRC, an area might be settled by people from many different parts of Britain, over time ironing out dialect differences. Communication between the major population centers was primarily by sea, so cities as far apart as Perth, Melbourne or Brisbane are in closer contact than they appear. Finally, there's the impact of mass culture and communication which tends to lessen regional differences within a country, especially for a country as young as Australia, where regional differences weren't strong prior to the introduction of modern communication technology.