HTC Desire -> Clove's £330 = NZ$715.74
Nexus One -> Google's US$529 = NZ$740.49
Shipping from the US cost me US$45 (DHL) + US$55 (comgateway.com)
Shipping from the UK via Fedex was (reportedly, IIRC) £45-ish
Overall, it looks like an HTC Desire from the UK is cheaper.....though the difference isn't huge: NZ$50-$100 (which isn't trivial, either).
The GST is definitely different. I'm seeing people here paying about NZ$100 while I paid NZ$149.50 for my Nexus One.