This is a bit late, but no-one has mentioned the traditional favourite for pack flotation: the tractor-sized inner tube. A bit smaller than a lilo when deflated and more comfy to sit in on the water, although you'll need two if you want to ride rather than swim. If the porterages between water holes are short they are easy to strap to the back of a pack fully inflated.

Those little half-length bicycle pumps can make inflation a bit easier without taking up a lot of pack space, but you'll need good biceps. The cheapie pumps for beach toys are lightweight and can be surprisingly effective, but are bulky.

Ortlieb make a lot of nice stuff, including a waterproof photo pack and various waterproof daypacks that could be used as a photo bag within a larger backpacking rucksack. I don't know how much gets imported into Oz, but the home site at www.ortlieb.de has all the info.