I lived in Brisbane for almost a year, rode a 600 down there as well. Brisbane has some remote, twisty roads that a lot of riders like to scream down. These are much like the California and Colorado roads, even though they don't have as tall of mountains.
The most important thing to note about the Australian death rate is the restrictions for new riders. You are not allowed to ride a bike larger than 600cc unless you have been riding for more than a year. Also, some of these guys probably know more about stunt riding than the squids in the US. I actually met the stunt rider who did the stunts for Mission Impossible 2. Just like Dan Jackson here in Kansas City, this guy broke his Femor doing something other than stunt riding street bikes.
I think the death rate really boils down to something a lot more simple than most would expect. Since kids these days tend to have a lack of respect for their elders, this bleads off to a general lack of respect. When you don't care about anyone else, you tend to be less concerned with your own actions. Take an adrenaline junkie, show him stunt videos and teach him to not respect others, and things will go downhill quickly. The Aussies tend to be a bit more reserved than people in the US, but maybe this trend is changing and the high death rate is a result.
Yes, I know, long post. Sorry