Remember that contrary to popular opinion, New Zealand does actually feature a basic form of civilisation which brings with it the almighty capitalist tool - the shop.
Leave out the little things that are easily bought out here instead, and if you end up having to leave out some clothes etc then it's not the end of the world... just go shopping if you find you need them! Just to add my experience however my bag was 31.5kg (it did have a whole xbox 360 in it!) and that got checked through without a problem. I'm not sure whether that was normal or due to my fellow colleague giving the Check in lady a cheesy grin.