Why don't you choose one that has approximately the same population and same population density -- that's why choosing an individual U.S. state might be a better choice because the U.S. is large and has a wide range of population densities.

The key point here is transmission of an infectious disease, so to get a proper comparison, you need to start with approximately the same probabilities of people infecting one another. That begins with proximity. Comparing the U.S. and Australia doesn't work because of that.