Pronunciation

So in chatting with one of my gracious Aussie hosts, he was trying to give me pointers on how to speak like an Australian. Apparently Aussies are really lax about saying their "R's" at the end of a word. For example: I would say dinner, in Oz you drop off the R, dinnah. And car would be cah. Work would be wahk. Yeah I'm going have practice, don't want to be labeled a tourist!
And why is it that Australian can do an American accent with such ease?