KInda makes sense. MEL-SYD is one of the worlds busiest air routes so they can tune the flights just so. E.g. if they remove a flight, there is enough empty seats an hour either side to take the people. If they upguage there is also enough people flying the return to keep yield high.
That said it is the main trunks like these with high yields that support the regional routes etc. Which are needed to keep pax who take two or more flight journeys on their network and not another airline.