To get from Auckland to Napier I had booked a domestic flight because that should take only one hour instead of eight by bus!
Do make sure that there is sufficient time between the arrival of your international flight and the boarding of the domestic flight. We ware slated to arrive in Auckland at 8:20, but we actually arrived around 8:45. Passport control, baggage claim and baggage inspection took until around 10:00.
It takes around 10 minutes to walk from the international terminal to the domestic terminal. There is a shuttle bus, but it is nice to have a walk after being cooped up in a plane for 11 hours!