Itinerary suggestions

The east coast is a party coast, so there is quite a bit of that going on (gap-year style drunken parties, dancing on tables, etc.) You can avoid it if you're not interested, but you'll probably encounter it now and again. There's lots of coastline (beaches, surf and sand everywhere you go), nature (wildlife, rainforest, great barrier reef, rivers, sand islands and dingos), adventure (skydive over Mission Beach get your adrenalin going enough?) and loads more to do and see. But you should be aware that this is the tourist/party coast, and if you want a different experience you might want to look at alternatives, such as 2 weeks in Tasmania or a month on the West Coast or perhaps time in the Northern Territory?

If you're not interested in a campervan - and let's face it, it's not for everyone - you could simply hire a car and do the same thing but sleep in hostels. Or you could use Greyhound, though you're less likely to get off the beaten track that way, so it's convenient for the usual backpacker stops, parties, etc. but less convenient for nature and smaller destinations.

This is peak season for Australia, so be prepared for crowds and traffic and all that comes with the holiday season. The northern parts of the country will be both hot and humid throughout December, so maybe a trip north might not be your cup of tea. Personally, id do Melbourne and surrounds followed by a coastal drive up to Sydney. The Great Ocean Road is something not to be missed, and Melbourne itself is a great place to be in the summer. Following the coastal route north to Sydney will give you ample opportunities to enjoy the coast and nature, as youve suggested. Sydney is equally great in summer, and probably a good option for Christmas-New Years Eve. You really need to look at your accommodation options well in advance, as the pickings will be very slim indeed. Having a car outside the cities would be good, but in the cities they become a pain. Enjoy...
Cheers, Andy...

That's a REALLY long way to drive in a month if you want to relax and see the sights. Also the petrol is going to sting the more rural you get. Id say you'd be better fly into Melborne, fly up Sydney, drive up from Syd through Byron Bay and the Gold Cast to Brissie, then fly up to Cairns - FNQ (Far North Queensland). Then you're only 100kms or so from Mission Beach

That's what I'd do.

Can you specify what activities etc you want to do? Nature, beaches, outlet-mall shopping, etc..??

Do you want to see the big rock?

Personally I think Melbourne to Cairns in a month is too much. You'll spend your whole time on the road. We comfortably spent 3 weeks doing Townsville to Brisbane and then flew to Sydney for a few days. That allowed for stops of up to 4 days in some places. We were there in winter, not summer, but didn't see any table-top dancing at all!

December in Cairns is very hot and humid i.e. very uncomfortable.
English tourists often seem to underestimate our summer heat.

dont listen to the idea of going to tamania, its really really not a good idea for a months holiday-so boring, unless your more like a 30 going on 55 kinda person. Stick to victoria and NSW, maybe NT if you are interested in seeing that side to australia. I would make sure i went to byron bay, sydney (bondi cant be missed), drive and go a few small towns on the way to melbourne, just stop offs but you will really get the sence of all things australian. Then to melbourne--best partying there (not so touristy and tacky as sydney-no offence) then spend a few days on the great ocean road for some more beach time! Further up north from byron is awesome if you can handle the heat Noosa is beautiful, it would be worth looking on the net at photos and seeing what takes your fancy. Good luck! you'll love it