open a bank account in South Korea

I would figure that in most countries, you have to be a expat living there first (since tourists can't just open bank accounts as one needs a residential address matching the country in question).

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).