I can't really tell if Australians sound like that... actually to me Australians sound like British XD but english is not my mother language so I can't tell...
What I can tell is that Robert is EXCELLENT at speaking accents, when I went to see AGOS I went with my aunt who's an english teacher and she asked me if Robert was british because he's accent is flawless. And of course Chaplin confirms it as well
Regarding Robert's facility with accents, I am always blown away by that moment in Two Girls and A Guy when he speaks the Hamlet soliloquy. It's as if he's channeling Laurence Olivier. Or maybe Kenneth Brannagh.
I found it weird but very interesting. Robert's character is what leads the movie and what makes it interesting but I found it weird how everything happens =P but definately one of Robert's best performances