Originally Posted by kiwibigdave

In practice you have this exactly right; the little planes can't fit cabin bags that size so you can check them for free instead. This isn't really a problem though - none of the big reasons you'd want to keep your cabin bag with you really apply on New Zealand domestic flights; things are extremely unlikely to get lost and the airports are so small they invariably come back out to you pretty fast too. I'd keep a laptop or camera on me just because they're fragile, but if you're really worried about not having your cabin bag with you at all times then you need to set the recline on your expectations to 'laid right back' for the duration of your time in NZ