Moving to new zealand can i take my iphone 4?

Can i take the phone i have and switch to a new sim over there?
or do i need to get a new one when i arrive?

Yes, you can.

We went for a holiday in NZ last year, and I used a locally-bought sim card for two months in my old Nokia. No trouble at all.

(We had a great time. I'm envious of your move. Enjoy!)