Australia is very marginal for the era covered by EU4. I think the overall province count is fine, certainly less of a priority than fleshing out some regions in the interior of Africa. Also, I think it's legit to have coastal wasteland, as some regions close to the coast (such as the Nullarbor Plain and a stretch of the western coast of Australia) are very dry and wouldn't have been suitable for sedentary colonists. Even crossing such a region by land and not dying of thirst along the way would have been a difficult undertaking for foreign colonists.