Phwoar, well if you were looking to split them off into the most geographically sensical units using the existing states, the best you could do imo would be Western Australia (obviously), Queensland + Northern Territory, NSW + Victoria + South Australia, and an independent Tasmania. Here's what you get:
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However that wouldn't be very plausible as the Northern Territory was historically a part of South Australia. So maybe an independent South Austalia including the Northern Territory would make more sense:
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The issue is of course plausibility, and in my mind that means economics. Tasmania historically had a strong economy despite it's small population due to it's fisheries, wool and location as a center of shipping; this of course waned in comparison to the other states as they developed and expanded in population but if the balkanization was early enough they might think themselves capable of going it alone.
NSW and Victoria will always be strong enough to be independent but I just can't bring myself to believe that they'd be independent of one another; they're too similar geographically, demographically and economically for it to make sense. So I always consider them as one unit, siblings if you like.
Queensland had a strong plantation economy that was actually cuckolded by the White Australia policy due to their utilization of Pacific Islander labour. So if the other states want to bring in that policy and their plantation economy is large enough and sugar prices are strong they might decide to abstain from acceding to the Commonwealth. Herein, perhaps they'll compel Britain to transfer more of the North Australian coastline where it was most tropical and appropriate for their plantations.
Western Australia was very underdeveloped for most of it's history, ditto for South Australia, so if they don't want to be dominated by Victoria and NSW they might decide to stick with Britain until a later date. All that gets you this:
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