I've done that journey quite a few times...
If you're flying air NZ, the flights usually get in around 5am (not so good). As others have mentioned, flying westward (longer days) is generally easier than flying eastwards. (unfortunately, the cheap flights usually go via hong kong/shanghai)
I'd suggest: mild sleeping pills (even a couple of hours sleep on the flight will make a BIG difference) (don't mix with alcohol, usual warnings apply, etc)
If you can, try to stay awake the day you arrive until it gets dark at least.
if it's sunny, try to get out into the sun, it'll help reset your body clock
- however, the sun in NZ is *very* strong! if you don't wear sunscreen/hat,et c you will *BURN*. (the burn time can get down to 8 minutes in summer!)
Expect to be tired for the first few days (you'll find yourself getting tired quite early in the evening, and waking up early.)
If you are going cycling there, you'll find the roads nice and wide, and (relatively) smooth, but the drivers are pretty ignorant of cyclists...