The best things in life are free
Americans: I know you are used to being able to spend your money everywhere you go, but do not count on being able to do that in New Zealand. Fortunately, there are money changers everywhere, including the airport(s). Using a money changer is usually a little cheaper if you're changing >$500 NZ (around $400 US), but they are still expensive (especially at the airport).
Instead, I would recommend bringing an ATM card and a credit card. Just about everybody accepts credit cards, but it is a good idea to have some cash for those that don't. I would strongly suggest that you ask your bank and/or credit card issuer how much it will cost to use their card here before leaving home.
Last I checked, you can use a Bank of America ATM card at Westpac ATMs at no charge. USAA should allow you to use any ATM for a 1% transaction fee.
Visa and Mastercard charge you a 1% transaction fee for all credit card purchases made outside the US. The credit card issuer will likely charge you something on top of this. If you have a USAA card, you will pay just the 1% Mastercard fee. Capital One is even better: they will cover the 1% charged by Mastercard/Visa...and if you have a cash back card, you can turn a profit of the transaction.