You've never been to Australia, mate. Australia doesn't have as many "enemies" as America... and it isn't as likely to get nuked off the planet in WW3. All said, thousands of miles of ocean separating Australia from the rest of the world means that military threats can only come from heavily industrialized nations with substantial military forces. Australia tends to be more "out of sight - out of mind" and generally not considered a threat to any other nations.
Don't forget that if you are holding Australia in a game of Risk, you will receive two extra armies per turn.