Correct, if you're at the highest tax bracket the extra burden would only come if they raise it all the way up to the 50% spoken of, expected to happen in the seventh year, as our highest marginal tax rate is only 45% (and I presume you are likely at the highest rate).
But then it does make ya wonder in that scenario if the ATO might be empowered to "clip" something extra if they're getting nothing from US dividends and interest at year 6 with 45% and year 7 with the 50% withholding rate. But yeah it's hard to know how exactly it would play out. Even if they STARTED raising the extra 5% per year over 15% we currently have withheld it certainly doesn't mean the administration after Trump would continue in like fashion all the way up to 50%. Some consider it just a bargaining chip.
I welcome anyone to correct me on any of that, but I understand even the experts have differing ways to interpret this bill and how it might or might not impact Australians, if it passes.
(This is not tax advice.)