Australia, New Zealand & the South Pacific - Freelance NZ over Xmas/Newyears doable?

I'm flying into AKL on the afternoon of 12/23 and leave in the AM on 1/2. I was hoping to rent a car and head either north to Bay of Islands or south to Marlbrough country and explore from there. Probably low key, some boat trips, wine tasting, exploring some more unpopulated corners.

Is this realistic over this holiday? In particular will I need to book rooms and a car right now? Can I hope for some flexibility if I decide to pick up and plop down somewhere else, or is the country out and about in force?

My last visit to the South Island for a couple of weeks centered around Queenstown and worked out well. Stayed with locals and followed my nose. A great two weeks.

Any hopes of something similar this time in the areas I noted above?

The traditional holiday spots (eg Bay of Islands, Coromandel Peninsula, Mount Maunganui/Tauranga, Taupo, Nelson) tend to be extremely full from boxing day (December 26th) to just after New Years Day. In those places, if you want to be sure of getting accommodation and rental car I'd definitely book beforehand.

OTOH main cities tend to be quite empty in that time - eg Wellington and Auckland.

Following your nose can be a great way to see the country, but it is unfortunate that you have picked the worst possible time of year to do that.

Kiwi Flyer is spot-on with respect to it being the busiest time of the year, but that alone wouldn't stop me giving it a go. I would definitely pre-book a car, and if I wanted to make it Marlborough and not the Bay of Islands I would definitely pre-book getting across Cook Strait.

As to accommodation, if you don't want the military precision itinerary that pre-booking will require, another good thing about NZ tourist centres is that even though they get to be busy they invariably have a 'Duty Motel'; based on the concept that a bunch of moteliers get together on a roster and whoever is on duty that day will manage the spare capacity in the area. i.e. You only have to find the Duty Motel and they will try to find you a last minute room. No guarantees, but the idea just seems to work. Though presumably better if you're on your own or a couple than a family, and have a little flexibility in your wallet.

And of course if you're prepared to drop standards a few notches and put in the effort late on any afternoon you might be stuck, last-minute options are much greater if you include youth hostels and campgrounds.

My last visit to the South Island for a couple of weeks centered around Queenstown and worked out well. Stayed with locals and followed my nose. A great two weeks. Any hopes of something similar this time in the areas I noted above?Absolutely, but it will be harder at that time of year.

I'm touring NZ with my wife also around that time- we arrive in AKL 12/25, and as of now, plan to depart AKL 12/31. (might push that off a day or two if possible) We want to spend most of our short time on the South Island. We have nothing else (car rental, hotel, or internal flights) booked as of now. Any advice in terms of timing? When should we buy internal flights? Book hotels?

If your dates are relatively firm, book now.