The one consideration is this: Aus has 22.6m people. The usa has 311m. At some starge your valuation potential will be constrained by adressable market. If there is an obvious niche that is too small locally, for example, you may want to consider it. But could still easily proof/concept and test product/market fit etc. And then compete on the terms of your initial success. And there are all kinds of examples (rocket internet etc) for geography being sustainable barrier to entry. Or of fast follow building a better product (FB vs myspace, etc. iphone vs palm). The variables are the potential for variation in the design/execution, the size of the market, and your own vision/skills, etc. Provided you have access to capital, talent, and a toe-hold demographic (oz is pretty good in ABC1s), why not start.