My experience with credit cards only
We spent three days each in Sydney and Auckland and I secured my Australian currency before I left. We discovered that the only time we needed cash was for the special shuttle arranged by our hotel ($16 AU pp one way to the airport).
We did absolutely all our purchasing in New Zealand with a credit card. Our only hiccup was at the Auckland museum when we needed a $1 coin for a locker, but customer service told us about the free ones available on the second floor.
Our capital one card with no foreign transaction fee worked great on this trip.
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