I'm a bad judge of accents to be honest. Even though I've lived in England, Australia, and the Netherlands, I just don't really know how to judge it.
I can't say I've heard Australian accents go wrong all that much, because, well, not many people try to fake them. I don't think I've ever seen a film where a person with an Australian accent wasn't actually Australian.
Though I will note that the most common depictions of Australian accents outside of Australia aren't accents that you actually come across all that often.
Meanwhile, English accents are another matter entirely. What I notice in American films is that they tend to pick accents which are, again, quite a bit less common than you'd think.
I live in the southwest of england, at the moment, and actually one of the biggest things here is that the accent I hear most in day-to-day conversation is one that you almost never hear in fiction (British TV being an exception, for obvious reasons).
I guess it's just the most boring of accents, since it's neither representative of any of the regional accents associated mostly with working class people from particular areas (scousers, cockney, Liverpudlian, and so on), nor with 'the queens english' (aka Recieved pronunciation, which tends to be associated mostly with the upper classes.
(I personally know all of one person that speaks with an RP accent, and that's because he went to a private boarding school quite a while ago.)