I imagine you could do this with a POD in the ARW. Say the British win, or the war is avoided altogether - with a stronger focus on America, and pushing west, I would expect to see the British focus on the rest of the world minimal at best. They had claims to Eastern Australia around this time, but there's no reason those claims couldn't be given up;
Fastforward to German unification; Australia, under whichever government, is underdeveloped and thinly populated. Germany, looking for a 'crown jewel' on par with British India, either purchases Australia from whoever owns it, or sets up its own claims, depending on the situation. The South Pacific wasn't really claimed until the 1840s+ anyway iOTL.