Qantas Cash or a OneSmart / Travelex (they're the same product but Air NZ use the white label product which is an inferior offering) don't quite work the way you think. You take an instant hit when you load the money to the foreign currency, which in the case of the OneSmart card is worse than the 2.5% you'll get stung just using a credit card. The rates on the Travelex card are better than the OneSmart because Air NZ take their commission from the product and use it to fund the Airpoint earn.
I haven't paid a cent to buy cash from ANZ for years as they give it for free to Platinum card holders and also have pretty good rates. I'd assume most banks give their higher value customers fee free forex.
My strategy for my last ~15 years or travel has simply been to take some cash with me and use my NZ credit card for purchases. I have however loaded cash onto travel cards so I can get cash from an ATM at times, but overal I find the time and hassle of doing all of this simply isn't work it. I'm more than happy to pay ~ 0.5% more just for the convenience of using my NZ credit card.
There is also a comment above about buying things in NZ$ on websites and the exchange rate being very good. I've come across a number of such examples lately, and one major one was United Airlines. Buying at their advertised NZ$ price was pretty much a direct conversion at the midmarket rate. If I opted to buy in US$ I'd then have to pay the extra credit card conversion fees. Some sites seem to be pretty competitive these days, others like PayPay are still pretty uncompetitive.