The suggestion to talk to an accountant on the subject matter is clearly the best advice.
I've lived in the U.S. for about 17 years now, and I'm thinking of FIRE within the next 2-3 years and return to Oz (will depend on the exchange rate and how much I've managed to cobble away).
My guess is that Australia and the U.S. will have a reciprocal agreement on taxation of these accounts. So if you have already paid taxes either putting money in, or taking it out under the rules of the country the money is in, then I suspect you won't get double taxed.
For me the worry is the other way round . . . I have tax advantage U.S. retirement accounts where no tax is paid for deposits (and in fact, the income used is tax free), and then you pay taxes when you make withdrawls in the future (ideally under a much lower tax rate). It would be a bugger if the ATO gets me on those.