They convert the cruise fares and port taxes at a fixed rate that Princess resets from time to time. They do the same for Canada. The Canada premium rate now is 1.35. a $1,000 US voyage costs $1,350 CAD. Once a guest has a booking, anything they purchase via Cruise Personalizer or App is also converted at the 1.35 (even if Princess adjusts the rate for new bookings). OBC is in USD for Canada.
I am sure in Australia, purchases pre-cruise from CP or the App will be at the rate used when cruise was booked. Their OBC is in AUD and remains as such for their cruises where ship currency is AUD. I am guessing an Aussie who gets SOBC and books a US voyage, ends up with $100 US from their $100 AUD.
For Australia, they take whatever OBC you get in USD and convert it 1:1 into AUD. Nasty for us, but works well for the Aussies coming the other way. This is things like MOBC, SOBC.