Genetic similarity comes about by having a common ancestor not so far back. Genetic radiation (the splitting up of that ancestor into different species) comes about by the exposure to each species-to-be to a different set of environmental circumstances that drive natural selection in different directions. The correct term is actually "adaptive radiation". Different sub-groups evolve to adapt to different environmental circumstances. In short, you would actually expect the frog to look different (as a generlisation).
Different frogsin widely separated regions can evolve to occupy somilar ecologicalniches. so a reed dwelling frog in Victoria and a reed dwelling frog in WA can end up looking almost identical. This is known as convergent evolution. However, given the stock from which the two species evolved were far apart terrestrially,they are not likely to be very closely related. So you can end up with two species of frogs that look almost identical but are genetically very unalike.
It's not the looks that count, it's the genes. Check out a phylogenetic chart (diagram showing genetic lineagesI for Oz frogs and that will show you how closely related or otherwise that the two species are. Without seeing one first, my guess would be NO.
Hope that helps rather than confuses.
Blue