We have done lots of travelling with our 3 children- both in Australia and o/s. Most recently: Melb- Cairns, Melb- Queenstown last year. We are going on a long-haul trip at end of May- to Europe, mainly Scandinavia where DH is from via Hong Kong and Thailand on way home... we'll be away for 5 months with lots of different weather so will have our own challenges!
My tip is this:
Buy some of those 'travel pods' that Kathmandu sell. They are mesh on one side and that travel bag 'fabric' on the other. with a zip all the way around. They come in black and a few different colours (lime green, orange, blue) and different sizes. We stock up when they have a sale on as they sell them then for 50% off (works out around $10-$15 each dep on size). Looks like you are about to go soon: foa anyone else interested, they have a big sale at Easter..
So, we have a couple each and we usually pack one for each child with just underwear and pyjamas and toothbrush. Then when we get to our destination, no matter how late it is, we just throw one each on the corresponding child's bed and we can easily find their pjs at least. We go as far as putting those Stuck on you iron-on labels on the kid's so we easily see whose is whose (and DH and I have larger ones in different colours). They pack really neatly in suitcases or bags and you could have say 2-3 each with different clothes in them. we also have some shoe bags from same shop so you don't have dirty shoes in the bags and a big long mesh bag that we use for dirty laundry. Last time we went to Europe several years ago we had one baby with us so I used one for nappies, wipes, spare clothes and bunny rug for the plane as it fit neatly under the seat without having to haul out a big bag from the overhead lockers every time we needed it.
Hope this helps someone- it has changed packing forever for me (esp since having 3 children so no more fishing around in a big black hole for that elusive clean pair of undies!)
All the best for your trip to NZ. We only had a week in Queenstown but it was sooooo unbelievably beautiful- love love LOVE New Zealand. Have fun