Yep - Invercargill.
Go to the website and get the real info (http://www.customs.govt.nz/features/charges/feetypes/Pages/default.aspx)
Zero duty on headphones. 5% on amplifiers. It's actually quite reasonable. Where you get hit is on GST.
As long as you are under (on an 0.80 exchange rate USD/NZD) $400 NZD - then you get no GST or other tax. Once over that, it starts at around $107 NZD extra ($60 GST + ~ $47 IETF fee). Obviously I can't publicly suggest getting around these fees - so for now, use common sense (keep your purchases generally under USD 320 including freight).
Hope that helps.
Thanks! I got hit big time with my DT880s. I bought 660s and 880 at the sae time and was making the order in a real hurry - saw there was no import tax fro the 660 so thought "yay no import tax" without seeing that the beyers had $90 import tax! It was only after I clicked confirm payment that I realised; then I was angry at myself for weeks lol.
But the bigger problem I found is finding things that actually ship to NZ.