Immigration can be pretty quick through the egates, presuming you can use them. It's luggage that can take a while. If you are not aiming to take anything declarable in, customs in my experience is generally 5 minute queue to hand in your customs declaration. Then 5 minute walk to domestic connection ("seamless transfer") desks where you need to get to at least 45 minutes before your flight. I have seen a 5-15 minute queue for check in quite often there (then quick security and bus straight to domestic gates). The benefit of a true connection is you can use bag drop rather than check in. I am risk averse and wouldn't want to pay a walk up fare unless you really must get to MEL as soon as possible. I would book 3 hours and enjoy a shower and coffee in the domestic lounge. With 2 hours and luggage you would be unlikely to have time for a shower and no leeway for delays to the incoming flight. Note you can't book 2 flights on the same day with QF to hedge your bets, they'll cancel them.