PIZZEY AND KNIGHT: Edition 9
In my opinion, this is the most up to date field guide and the illustrations are superb. I use this as my "shelf book" where I keep all my references etc.
I find it a little too big for the field but if I were only to purchase one field guide, this would be it.
MORCOMBE: Compact Edition
This is the guide I do carry in the field. I find it very easy to navigate through and it fits comfortably in bags and more importantly, back pockets.
MORCOMBE: eGuide - iPhone and Android.
This is the device application version of Michael Morcombe's field guide.
It is excellent not only for the portability of the field guide but because it has built in sound recordings for the majority of Australia's birds.