Australian dollar is a weak currency for international travel I've heard, as the exchange rates are always lower than referencing from USD (like for example, A$1 = 0,60€ making it expensive). Like not all countries accept AUD at face value while USD is literally recognized everywhere.

Basically, they would have to pay more. Don't even talk about currency exchange with AUD, as you will get less than USD holders, for instance: A$100 = 60€ while $100 = 87,20€ (A$146), which is why refrain using Australian Dollar for international travel (just bring US Dollars instead).