The exchange rate on the Air NZ OneSmart is actually a lot worse than the Travelex card despite it simply being a white label offering. The rates for all the white label offerings (a number of banks offer this as their travel card) are worse than the Travelex rates, and this difference is essentially the margin they make on the product.
In the case of the OneSmart card Air NZ giving Airpoints Dollars on spend, so they’re needing to make margin to offer this back to you… You’re really just paying extra for those Airpoints Dollars that they give back to you!