Whenever we've been to FNQ (Far North Queensland, flying into Cairns), we've always stayed in Palm Cove and not Cairns .... except once, when we had some Hilton Points we figured we might as well use. Our two day stay in Cairns re-inforced why we always stay in Palm Cove and not Cairns.

Opinions will vary but Cairns (imho) has nothing to recommend it (unless you like pubs, clubs and McDonalds and are looking to party). Palm Cove (about 25 minutes up the coast) or Port Douglas (about an hour) are far nicer places to stay. Cairns itself doesn't really have a beach.

What are you planning to do? A day out on the reef, a day to take the train up/skyrail back to/from Kuranda and a day out on the Atherton Tablelands? That's three days taken care of - what are you planning for the other three? Do you want beach time? Sit by the pool/spa time? If so, you don't want to be in Cairns.

I would be inclined to limit the time in FNQ to four days and spend the time elsewhere. You should, of course, be coming to Melbourne, one of the world's great cities....