747 carries 25 extra business class seats over the 777-300.
Thats $87.5k extra revenue per flight. Daily return flights = up to $64m pa in extra revenue.
Using your Mel, Syd, Bris example would equal up to $192m per year in extra revenue.
Thats before even considering that the 747's have no remaining capital value whereas the 777 would cost as much as $300m each which needs to recouped. Lets keep it real simple, 6 777, 15 year life = $120m in capital costs per year that need to be recouped on a straight line basis ignoring the time value of money.