Just pay Customs directly once you get your notice from NZ Post. They accept all payment types and have an 0800 for credit card payments. It's all here:
http://www.customs.govt.nz/inprivate/sendingitemstonz/Privateimportsbypost/Pages/default.aspx?s=5
LostBoyNZ: Last time I was stung with the $75 NZ Post customs fee, on top of the regular GST and such. This time I asked them how I can avoid that, and talked to Customs this morning. They said once I've received the letter from NZ Post, to take it to my local customs office (near Christchurch airport here) with my letter and the invoice from the products, and I can pay them directly.
I guess I can't do it over the phone then with customs and my credit card? I'll give it a try when the letter arrives though.