veritas28

FWIW I reached out to an Australian who did their PhD in the US and s/he never declared their stipend to the ATO and there doesn't appear to have been a problem so I may have been mistaken in my initial assumptions.

I think so, yes. Any amount of a scholarship/fellowship beyond 'qualified education expenses' (tuition, direct course costs) is subject to US income tax and Australian residents are taxed their worldwide income which, by definition, would include the amount subject to US income tax. Scholarships/stipends aren't explicitly mentioned (neither as assessable nor exempt) but I'm assuming they are taxed in Australia by virtue of being taxed in the US. It's interesting because the ATO saw nothing of my APA but they'll gladly siphon off my non-Australian stipend that has nothing to do with Australia.

Yes, there are! Quite apart from ongoing academic considerations, I'm currently most concerned about receiving an adequate stipend given 14% of it will be withheld because of US income tax. Add to that Australian income tax to be paid each year, albeit with the foreign income tax offset, and it might make ends slightly harder to meet than the initial stipend figure would suggest. It doesn't dissuade me from going to the US but it does make finding non-US sources of funding all the more important given they would be taxed neither in the US nor (I think) Australia.