I guess it's a pain to find them in Australia, but you really need to try them on to see what fits. Sounds like you are inbetween a US 11 or 12. Pretty common sizes to stock out here. Probably because we are a bunch of fat ass tubby bitches in the US. Aren't boots offered in half sizes for you guys too? Sounds like to me you're an 11 1/2 US. (Splittin' the difference, I am a genius like that...)
If anything, you may have to go with the larger size and have a shop add some volume filling stuff to get the right fit. Again, you're going to have to try them on. Even the same exact boots of the same size can fit differently.