If someone is between locations a lot, surely it's their choice where they wanna be enrolled, as long as they're only enrolled in one place at a time and only vote once. Anything less seems to be a contravention of the principle that people should be able to move freely and easily within different parts of the same country and have easy access to political participation.
As to the second, no, it's not a contravention of federalism to have a national body administering a national electoral system. It's an appropriate application of the principle of subsidarity - the appropriate function to be performed at the appropriate level, meaning the central and national body performing tasks which cannot be effectively done at a lower level. Administering enrollment and national elections really should be done by a national agency given that they affect, in a uniform way, the entire country's politics. I mean, when you have people attempting to rig enrollment and residency rules, for partisan gain, at a local level, that must tell you something is wrong with letting small areas (often politically homogenous ones) do their own thing.
Further, in keeping with federal principles, enrollments are still separate at State and Commonwealth (federal) levels. When you change your enrollment with the Australian Electoral Commission, it also automatically gets sent to the relevant State body so you're enrolled for State elections. Both levels of the federal system work together! Quite convenient really.
(Interestingly, you guys generally have both levels of elections on the one day, on one ballot even, voting for state and federal politicians at the same time. How is that condusive to the separation of state and federal powers and politics and issues that are supposed to exist under a federal system?).
So yes, states still run their own elections for state parliaments, with quite different electoral systems. They draw their own boundaries for state elections, run state elections themselves. But, enrollment is simple, seamless and uniform across the entire Commonwealth. Voting in Commonwealth elections, for the national parliament, works the same everywhere. Electoral districts are drawn according to uniform, nonpartisan principles at a national level. The same postal, pre-poll and absentee rules apply everywhere. In short, people moving around within the one country don't need to figure out the local arrangements for enrollment, residency, eligibility, etc, to be ensured their democratic rights in national elections.
Anything else just creates needless bureaucracy, duplication and inefficiency for no actual demonstrable gain.
