When I hear Australian's doing American accents, and I mean the big names like Mel Gibson, Russell Crowe, Nicole Kidman it's always slightly cringe-worthy. I'm Australian but as well as English and Australian programs we grew up with a hell of a lot of American TV and especially movies. I have a flare for accents and have a good ear for listening to how people speak.
Anyway, the real reason the better of the Australian actors end up in Hollywood is because they make a film which gets international recognition. And that opens a door for them to get work in American productions. Nicole, Mel and Russell got breaks with Dead Calm, The Road Warrior (in Australia it was called Mad Max) and Romper Stomper.
Even now days the state of Australian TV and cinema production is one of a very small industry. Basically America does TV and cinema better than anyone due to more experience doing it. The production values are higher, the talent pool is much larger, the budgets are higher. Australian movies always come off as a bit of an amateur hour affair by comparison.
Also the Australian accent is really quite broad, and not one that Americans or in fact most of the world hears constantly on the TV. Neighbours has a huge UK following so that is one exception, but that's just one half hour show five times a week.
So it's very much more natural for an Australian to mimic other accents as we all grew up hearing them, rather than the other way around.
Just my thoughts, not only to TxMike's comment but in general to this thread.
This movie felt very comfortable for me, in terms of, it wasn't an embarrassment to watch. But consider the two leads, they've each spent enough time around Hollywood to get their acting chops down and know what a proper production is all about.
Scientologists love Narnia, there's plenty of closet space.

That's probably the most accurate answer. Foreign actors have to learn an American accent otherwise they wouldn't get many well-paying jobs. The reciprocal isn't true. And believe me, many of the "American accents" done by foreign actors are NOT very authentic, but they try!
.... TxMike ....
Take a risk, Take a chance, Make a change.
Kelly Clarkson - Breakaway

Because most other countries actors can act better than most American actors. HouseEnglish. Three of the main characters out of the WireEnglish. I could go on. America has plenty of Hollywood glitzy celebs like the awful Tom Cruze and that annoying woman out of Fiends but very few accomplished actors. Plus other countries can do American accents no sweat. Americans can do Dick van Dyke. Owight Mawy Poppppins.

Why would an Australian production for a film set in Australia cast American actors? This is an Australian film. You're either a troll or incredibly dense.

However, while she may have been born in Honolulu, her parents were both Australian visiting the USA on Education Visas. She grew up and was educated in Australia.

Nicole Kidman was born in the USA.
.... TxMike ....
Take a risk, Take a chance, Make a change.
Kelly Clarkson - Breakaway

Who was the American? I saw all Australian actors except for Joseph Fiennes.

I wasn't aware that casting should conform to affirmative action.