An important part of any civilisation’s description is both what form its centres of power/wealth take and how they are spread out. The Germanic barbarians that the Romans fought had almost all their power (specifically military) and wealth locked up into small, spread out, villages, they had towns but their destruction would have been of little consequence. While the Parthians had most of theirs locked up in towns and seminomadic settlements (mostly in the Iranian foothills), the capital and other major cities were sacked quite often without breaking the Arsacid’s power base. If your wondering if that had anything to do with the limits of the Roman empire, the answer is yes it had a lot to do with the empire’s limits, since the Romans could conquer any city centred society and the Roman empire was city based for this reason. The main difference between the Roman empire and say, Renaissance Italy was that Rome was the major centre of power of the entire empire while Renaissance Italy had multiple competing centres, one of which was Rome. Australia’s case is closer to Renaissance Italy than Rome, with the addition of a political capital rather than a religious one, Canberra, which isn’t an economic, military or cultural powerhouse. Contrast Canberra with London, Paris or Beijing and you’ll see how odd Canberra is as a capital.
The reason for this is that each state would be a separate country if they where in Europe, either by size or population, after all Victoria has the same population as Denmark and the South Australia has a missile test range the size of England, it used to be twice as big. The main difference is that while most European countries have several big cities, Australian states only have one. So, when it came around to needing a political centre, neither Sydney or Melbourne (the two biggest and wealthiest cities at the time) were willing to let the other be that centre and they obviously didn’t want a new city which was economically powerful enough to be that centre. Thus Canberra, a town without any of other method of becoming nationally important, it’s not even on the coast, became the political capital. Canberra, unlike most capitals, isn’t a centre of power or wealth and this has one important property. Unity is a lot easier since each state isn’t competing directly against one another, with one being the centre and the rest peripheral but instead on neutral ground. Put it like this, if Edinburgh has a grievance with England, its sorted out in London, which is England’s centre and that’s a large part of Scotland’s drive to Independence due to the disadvantages imposed on Scotland by this arrangement. If you had an equivalent argument between Adelaide and Melbourne, it’s sorted out in Canberra which isn’t any states centre and while Adelaide is at a disadvantage because its smaller (like Scotland), Melbourne doesn’t have the home ground advantage London does. It also helps that Australia is a single Nation (i.e. a nation state), as opposed to the United Kingdoms, which is several nations in one state.
An important consideration is how Australia’s power distribution change due to Overshoot. Since this is a question of distribution, instead of absolute numbers, a decline in population won’t affect power centres unless it includes relative changes. Most Roman cities survived the collapse and continued as power centres, London and Paris were both around back then, even if Barbarians took over because that was just a case of out with the old boss, in with the new, same as the old. The key thing to remember about cities is that they only require about 5-10% of the population to become the main power centres, economic and political rather military, except in a few cases (today and Rome after Gaius Marius’s reforms). Parthia’s military might was centred around its dual use of horse archers and what eventually became knights (yes, knights were originally middle-eastern), without being nobles, who were recruited from the Iranian foothills, destroying the cities would have weakened this army, because it would have been harder to fund and equip but wouldn’t have broken Parthia’s military. For the Western world, the process has run towards centralisation more often than not, after all Germany now has cities and many of the people the roman’s conquered didn’t have cities during Alexander the Great’s time. In the Middle-East the process has run in the other direction more often than not as the fertile crescent has become less fertile. The entire place used to be filled with city states (like Babylon) and while there are still cities, their importance has dropped. Centres of power don’t normally shift rapidly and when they do shift rapidly its only in response to quite rare events, such as the various diseases which wiped out 90-100% of the Aborigines and Native Americans, otherwise it takes centuries or more often millennia to change.
What is likely to happen in Australia is that cities will remain as the centres of wealth and power, but that the current extremes in the capital cities will disappear. While the current capitals will most likely stay the capitals, the other cities (such as Albany, Launceston, Newcastle or Bendigo) will gain in relative importance as well as housing a greater proportion of the urban population. Most of the social and physical technologies necessary for a city based society are present in highly resilient forms e.g. Boats, farms, roads (not necessarily for cars), granaries, a working justice system etc. The main difference from most historical examples of city based societies is the greater distances between our major cities and all the major cities are on the coast.