I was on a cruise in NZ late January/early February. It is indeed true that everything imaginable can be paid with a card in NZ and Australia now, even a single cup of coffee. The one thing that we did need cash for was buying home made garments such as wooden beanies, blankets and jumpers from the shore excursions to sheep farms. We had about $40 NZ from a previous visit to NZ, but the beanie I wanted was $50 NZ.
However, your problem would probably be what you would do if you had that sort of money left after your cruise, with no intention of visiting NZ again for many years.