Well, I finally went out and took all my stuff with me and selected an equipment bag. In the end, it was not the one I was originally going to get.
I looked up on the net suitable bags, and found a local shop who stocks them and took all my gear that was ging to be packed into the bag with me. Firstly, I just put 1 camera with lens attached into each of the bags and put them on. It was pretty obvious to me within a few seconds which bags would not be too comfortable to carry. The just wearing a bag with something in it test actually only left me a choice of 1, with another one a close 2nd.
The one I ended up getting was the Lowepro Compu Trekker AW, and the 2nd choice was the Mini Trekker. These 2 bags are almost identical in size, except the Compu Trekker is just slighty more square at the top giving just a little bit more internal room. All up my bag and gear comes in at 8kg - which is right on the limit for airline carry on luggage.
The first thing I did was to load all my gear into the bag at the shop and I then went out to try it. I went to the top of Mount Lofty (overlooking Adelaide) grabbed my bag and took some photos. I swapped between having each camera in my hand shooting and having it in the Backpack. After an hour of walking around with the pack on my back I was pretty happy with my choice.