Why. It has to be God's testing grounds or something because that sh*t is whack. The only normal animal in Australia is a crocodile and that's not a good thing. The platypus. The f*ck is it? Same with Kangaroos.
Even Australia's Crocodiles are the largest in the world. The Black Mamba and The Funnelweb Spider are some of the most venomous of their species. Emu's have a spike similar to a Utahraptor's claw. Sharks roam there a plenty. In the words of Steve Irwin 1962-2006, Crickey!
I'm not sure how. I'd get tired of checking the toilet seat or shoes for poisonous things. Not gonna lie though, you guys have a great national defense in the event in an invasion. But you'd have to be pretty dickish to try to invade Australia since everything there is looking to kill them. *Looks at Japan*