I have sampled at least a dozen gourmet and fast food burgers throughout Australia, and I will admit that I have not had the Mookie.
With that said, please see my post from the other thread regarding burgers in Europe. Australian burgers are just as bad, if not slightly worse. It's not just the beef that is bad (and it is bad,) but the nasty slaw that all restaurants seem to slop on the bun. Burgers and slaw are a horrible combination IMO.
Australia has some amazing lamb dishes/sandwiches. What they do not have are amazing beef dishes/sandwiches (burgers.) I would say that every burger I've had there was forgettable but then I would be lying. They were all memorable, just not in a good way. I am disappointed in myself that I have given them so many chances. Fool me once...
When in AUS or NZ I would just stick with the lamb. You almost cannot go wrong.