Oh gosh, I wish I had an answer for you! I can think of several reasons why they are popular here - they are a great way to get cyclists from all over the place together to ride; they are a wonderful fund-raiser for various charities and they are a physical challenge without it being a race.
As for why there are none in Australia - I had no idea there weren't any! Wow...so....someone needs to take the plunge & organize one!
"When I'm on my bike I forget about things like age. I just have fun." Kathy Sessler
2006 Independent Fabrication Custom Ti Crown Jewel (Road, though she has been known to go just about anywhere)/Specialized Jett