We went last Xmas for 3 weeks and had a fantastic time.
Helium is right though - you need to decide why you are going and what are your must dos and then plan around that.
This was our itinerary in case it helps
Flew Cathay Pacific from London to HK to Sydney - we decided the main aim of our holiday was to see Australia so we decided not to stop of just keep going. It wasn't as bad a trip as I thought it was going to be ( we have 3 DCs who were 8, 6 & 2)
We stayed in central Sydney in an apartment hotel for 3 nights and saw the (child friendly) sites, Taronga Zoo, Aquarium & Darling Harbour (on mono rail), Sydney Opera House (didn't go in), the bridge and the Sydney Eye. One thing to bear in mind is that it is vvv expensive at the moment so take plenty of spending money.
We then picked up a camper van and started our journey down the coast to Melbourne over 10 nights staying in camp sites along the way.
One in Melbourne we stayed in a central Melbourne hotel and again like Sydney did the sites we could in a couple of days - aquarium, St Kilda beach, trams.
Then we flew to Perth to visit friends for Xmas. Boxing day we flew back to the UK the way we came.
It was a fantastic holiday and I would happily do it again.