Australia is the only country in the world where you need ETOPS classification to fly some domestic sectors. For example, the direct flight route between MEL-PER crosses the "Great Australian Bight", which has areas that are about 90-120 min from the nearest alternate airport (I do not know the exact rating required). The alternative is to fly around the coast, which adds time, cost and would actually cause range problems on some narrow-body aircraft.
This also means that you have the decision of how many of your domestic fleet do you configure to fly this sector. ETOPS classification is expensive, and if you throw in stuff like life rafts (cost of equipment, plus the weight it adds to the aircraft), then your costs will spiral upwards. MEL-PER itself is a very profitable sector for Australian domestic carriers, but those aircraft will need to fly on shorter sectors as well.
I also heard that to fly PER-DRW you need ETOPS classification, but I am not sure about this claim.