Kicking off my feedback and travel advice re a visit in New Zealand...
First things first, New Zealand is not a very efficient choice for a WHS traveller. The size of the country is way larger than most maps will indicate as it's on the edge of most maps and the scale is skewed. Roads are good, but not fast; e.g. what I would call highways only exist around Auckland. Most roads are country roads, some bridges are single lane. Last but not least, the inscribed sites are few (3), one of which is prohibitively expensive and even if you had the money (10k$), you would also need the time (a week on a cruise or more) and get a reservation (sub antarctic islands). In addition, New Zealand is not very active re their tentative lists with only one tentative site being scheduled for a WHC.
That said, it's still a great travel destination.
Unless you are based in the Pacific or Oceania, you will likely have to transfer. For Europeans, choices are China, Hong Kong, Singapore, Australia or Los Angeles. The last choice is discouraged as it requires a transit visa in the US. I opted for flying in via Singapore and Sydney and flying out via Shanghai. This way I am able to polish my WHS visits a bit (Singapore 2, Sydney 3, Shanghai coming up). Nowadays, there are plenty of long distance budget airlines operating in South East Asia/Oceania, so it is not that expensive to fly one way Singapore -> Sydney and Sydney -> Queenstown. Experience with Scoot and Jetstart were ... interesting, though. It's not what you would expect from a long distance flight, but it did the job.
I organized it such that I flew into Queenstown (South Island) and fly out of Auckland (North Island). This saves you plenty of travel time. It's also a good combination with a side trip to Sydney.
NOTE: Do not bring ANY foodstuff (biohazards) whatsoever to New Zealand. And clean your hiking equipment, specifically your boots. They will fine you heavily if they find you with dirty boots or an apple.
Getting Around/Rental Car
There are buses, but we travelled by our own car. New Zealand is a road trip country and you will miss out if you travel by public transport. Be aware that travel times tend to be way higher than you would assume. Those sign saying driving in New Zealand is different (i.e. slower) are correct. There are plenty of reasons for this:
1) There are no highways. You mostly drive what in Germany would be called a rural road. Overtaking a slow Lucy (camper van) can be difficult. Max speed is 100km/h.
2) There are plenty of road repairs all across the country.
A few advices:
* Don't speed. We saw plenty of traffic police in remote parts of the country.
* Get proper car insurance including glass damage. There are plenty of stones on the road. Preferably, you don't get it at the rental agency on site as they grossly overcharge you.
* One way rentals South to North are supposedly cheaper than the other way around. You should definitively check and see how you make arrangements.
* The ferry in Picton requires an early reservation. Most rental companies will collect the car in Picton/Wellington and give you a new car on the other side.
If you are wondering, why I am bringing this up again... Road conditions are a challenge and road repairs a constant feature of the country. It's not that the Kiwis go cheap on their roads, It's that these roads are subject to nature forces, including floodings, mud slides, ... Before our visit, tourists got stuck at the Fox Glacier/Franz Josef Glacier. The single highway of the area was shut both direction Wanaka (South East) and direction Greymouth (North) due to floods. I did see the impact the floods had, and there was no quick workaround. And if you are wondering, why didn't they just take a detour, you need to know, that the whole area only has this one road going in and out. So check the weather, plan with enough buffer and don't be stupid.
I would recommend to trust Google maps here. They tend to have the construction sites / road blocks. And seeing even the "big" highways are just normal rural roads it doesn't really matter where you drive.
Supposedly, December to January is high season. And there were a few places where we needed to prebook and had a hard time getting a cheap accommodation. However, high season felt pretty empty to me. The place was not crowded at all.
Like I said the Sub Antarctic Islands are prohibitively expensive. Problem is that the cruises always to the full monty. There is no 2? day tour to the Snares (the closest islands) only.
Of the two sites, I seen Te Wahipounamu clearly belongs in the 4.5-5* region. It's huge. It covers so many different parts: fern rain forests, coastal regions, pristine mountain ranges, fjords. It's remote and very little human interaction has happened. I have a really hard time understanding any rating below 4* here. Only complaint I can accept is that the site is too large and should have been split in multiple separate inscriptions to allow for a better appreciation of the different components and make the different parts more visible.
@Solivagant: If you give the West Norwegian Fjord 5* , then I really have a hard time understanding how you can award only 3.5* here. Milford Sound easily matches the Naeroyfjord.
Tongariro is a bit harder to judge. Thanks to some terminal cancer patient (a German) who had the Tongariro Alpine crossing on his bucket list and who died in the crossing, (Quote: My father was really sick, but totally in the condition to do an alpiine hike) we had to postpone our crossing from Sunday to Monday. The Maoris did a traditional mourning ceremony from Friday to Sunday for this irresponsible moron. Problem for us apart from the stressful drive to Auckland on Monday was that the weather was really bad and views were terrible. Unlike Sunday, where the sun shone. We did also explore the Southern side on Saturday which was nice. Still, my benchmark for volcanoes is the Teide and I feel Tongariro is not on the same level. I think 3-3.5 would be correct.
We only managed to see one proper tentative site, the Auckland Volcanic Fields. If there were more tangible Maori remains, I would be strongly in favor. But I failed to notice any. Best part was the youngest volcano, Rangitoto, which is a protected nature reserve. I would have also liked to visit the outside quarries.
We also ticked off the Fjordland site, but this is presumably an extension of Te Wahipounamu and to me a no brainer: The lakes and fjords are part of Fjordland.
Of the rest, both Abel Tasman and Napier sound enticing to me. Abel Tasman is further North than Te Wahipounamu, so fauna and flora should be different. Travelling in New Zealand I saw plenty of fabulous Art Deco and if Napier has a consistent set, it would be nice addition.
White Island has been in the news recently due to dead tourists. I cannot judge the site, but assuming the Eolian Islands are inscribed, I would think this makes sense, too.
But as mentioned, the Kiwis are not too eager to add new sites.
Personally, I think the Great Walks are great and unique to New Zealand. I think they could make a nice inscription. In addition, the New Zealand Gold Rush would be nice. Rotorua should also be added as it shows how Maori used the thermal energy. Last but not least, if there is any authentic Marae from before the 20th century that would be nice.