If flying first spa treatments qantas will call to book you a time for the spa and also as you have a long transit you should not have too much trouble to find a time.
Second point I would eat in the Flounge and then sleep on the flight to HKG obviously plane food has issues just because of the nature of where you are being served.
Yes you could go into Melbourne but it will take an hour each way,then you have to get back say 2 hours before your flight so I guess is it worth it for 4 hours in Melbourne. I would probably just relax in the lounge that way you don't have to stress about delayed transport back to the airport etc.
Flounge is really nice since you have 8 hours why not compare it to the business class lounge down stairs and you will see how much better the first class lounge is
Overall Enjoy and just enjoy the experience