Strange - I'm a long time resident of Sydney, but I grew up in "the bush" (ie not Sydney) - and I'm not at all familiar with it as a common expression.

Anywhere but Sydney is also called "the sticks" - even the outer western suburbs can be referred to as "the sticks". Being "a westie" is not regarded by those further in as a Good Thing. I am even pushing it by living in Earlwood and not Redfern, Newtown or Erskineville.