I found using Lebra sim cards was the cheapest all round as you can phone Britain for 3cents a minute and they're also cheap within Auzzie too.
My highlights were in W.A, 4x4 driving through the Gibb River Road was something not many do any was exquisit.
Also if your around Broome you can get a job in Malcolm Douglas's Crocodile farm as he takes backpackers in season work. That job for me changed everything.
I never found any scams on Gumtree but maybe I was luck. I used it all the time, from jobs, vehicals, travels buddies and appartments.
If you intend on staying in a city for a while then getting shared accomodation is far cheaper then any hostel. Or you can try couch surfing on http://www.couchsurfing.org/
Enjoy yourselves, its a life changing trip!