It's impossible to get a perfectly Australia-shaped Austria, but here's my shot: Austria gets a better deal out of WW1, while Germany is hit harder and ends up on the verge of collapse. At the 1919 Peace Treaty (Trianon, Versailles etc), the Austrian Army is still largely intact and the Imperial Government has held out in Vienna. They get to keep the Southern Sudetenland and are co-opted into helping the new German Government in putting down communist uprisings in Bavaria. Austria ends up occupying Bavaria while the rest of Germany fragments into successor states, and the 2 are locked in customs Union for a while before Austrians and Bavarians vote to merge into one nation by the 1950s.