If you have Sapphire status you can use the business class check-in at MEL and have access to the QC. Otherwise you have neither in MEL -- you do in SYD. QF is pretty strict about keeping the J checkin clear and will go by your QF ticket (you can upgrade it to J for a few hundred dollars which gets you checkin but not QC access). You'll have to wait until SYD to use the lounge unless you happen to meet a friendly agent who bends the rules a bit.