Most of the local magpies have a black back but there is one near home with a white back like this. The Simpson and Day Field Guide to Birds of Australia describes several races of magpie. Is this a local variant or an extension of range of white-backed ones?
Based on the map in the morcombe guide, ACT lies on the line where the ranges of 2 races, White-backed and Black-backed Magpies meet. White-backed occur to the south and Black-backed to the north. Much of the population are Black-backed or a mix of black-backed and white-backed. I suspect a mix is the majority, but I can't say I have paid enough attention to be sure of the proportion, it is not all that obvious unless you pay special attention. Pure White-backed magpies seem to be a lot fewer, but they really get your attention when you see one. There is a decent number of them on Wanniassa Hills, and aren't rare although less common.