Ranger Camping Stores have the BLACKWOLF brand which do cheap Duffelpacks (see http://www.blackwolf.com.au/engine/SID/ ... 102481.htm
They are BIG, good handles and broad shoulder strap. I reckon I could ride my bike with my 150L (900mm long) on my back. Only one side pocket (no big deal) and are probably not really waterproof - but then not much is. Use inner plastic bags anyway. Cost $39.95.
Mark it in some way as there may be a few of these!