There are no arrivals lounges in Australia

If you are connecting through Sydney , then you will have access to the Qantas Club before departure to Melbourne. If you are on a domestic flight to Melbourne ( QF400 and above ) , then you will clear customs/immigration, head to domestic transfer and take a bus across to the Domestic Terminals. You will then have access to the Qantas Club which has showers

If on a QF flight 1-399 to Melbourne, then you just pass through security in the international terminal and head upstairs to the Qantas Club which has showers

Dave