Yes he could use his Australian credit card but that incurred a "transaction charge" for each and every use abroad.
You can obtain a special card for travel that apparently reduces those charges. When I applied for one at my bank prior to travelling to NZ, the staffer I spoke to me gave me a very useful hint. "Just use the ATM's owned by [redacted] and you won't incur any extra charges; that bank is owned by this bank. But don't tell anyone I told you!"