Why are there so many great actors and actresses from Australia?
Virtually all Australian actors are formally trained professionals rather than celebrities. They have studied drama, singing and dance at NIDA or a similar acting school. They have worked in TV and theatre in a wide variety of character roles. This gives them a strong performance background.
In many other countries actors are often just a pretty or interesting face. They have no real training in acting. They just play themselves speaking lines.
My best guess is that smaller population versus say...the US means they have to put a lot more effort in to be seen to start with. That's not to say US actors aren't great there are several that are!
Just that the talent pool is smaller so in order to be seen they have to make an impact. That goes for any country with a smaller population.
Then again I'm Aussie so I'm a bit biased ;)
I'm sure there are a myriad of other reasons why, but from where I'm sitting this seems to be the case