Oh, that's easy!
14th of May 2004 Miss Mary Donaldson from Hobart, Tasmania marries the Crown Prince of Denmark. The couple soon get rather bored by the lousy climate in Denmark and buy one of the Solomon Islands, where they spend most of the time. That again leads to a mass immigration of Danes who want to be near their Crown Prince and Crown Princess. But most of them never get beyond the good bars in Perth.
After a couple of years many Australians are (even more) tired of the Windsors and look around for some more present royalty, and soon Frederik and Mary are candidates for the titles. This in no time leads to a gigantic bar fight all over Australia, the Windsor-loyals against the Frederik and Mary loyals. The Windsor loyals are of course in no time pushed back to NW Australia and Frederik and Mary are proclaimed Bruce and Brucella of Down Under including also New Zealand, Papua New Guinea and Solomon Islands. As a tribute to Mary, who really started it all, the NE part of the new Brucedom is called New Maryland, and the Welsh-, Scots-, Irishmen and Danes (from mainly Zeeland) who tipped the balance in the Great Barfight each get a piece of the country named after them. The Tasmanians are pissed over Mary not staying there anymore, and don't want to be called anything but Tasmania, and the old Windsor loyals are given NW Australia under the name Australia as a reached out hand for reconciliation.
Regards
Steffen Redbeard