Hello!!
I went backpacking to oz oct-apr so was there most of the summer. From what I have heard there are backpackers around oz 365 days of the year (so you won't be alone). Even though it is the winter, it will still be really good weather, with you being able to do things in much more comfort than in the summer. If you are there in the winter head for the snowy mountains and get some snowboarding in, there will also still be surf courses up the east coast (spend most of your time on that coast...it rocks!!). I went by myself but through a company called http://www.bunac.org/, where you meet a ruck load of people and they organise work visa for you (if you want to work there)
To be very brief I did the following -
Sydney - Byron (Doing a surf course)
Fraser Island
Whitsundays
Cairns
Red Centre
Great Ocean Road
Melbourne
I missed out the west coast, but I do intend to go back there someday to experience the 'real' Australia.
Enjoy it!!!!!
Mark