Looking at putting money in Roth IRA and Roth 401k. Partner is Australian so we might retire in Australia. However, I have worries that the money will be taxed in Australia instead. Any people have looked into this? I guess people who retire in Canada/UK/NZ would face similar issues.
You need to find out what the tax treaty with Australia says. The answer is different for each country based on its own tax treaty with the US.
OP were you able to find out?
Canada treats Roth as being non-taxable in any scenario where it would be non-taxable by the US, not sure if Aus is similar.