I think it may be a matter of how many Aussie actors who work overseas assume other accents to fit into the setting they are playing - there's a good few rather well known Aussies around, both in TV and movies, who do mostly American characters so we don't usually hear their Australian accents.

So, outside Australian productions, Aussie accents are rare things in entertainment. Plus Americans are rarely ever good at imitating Aussie accents.