I promise to not use the phrase "the land of the long white cloud"....
Having only lived here in NZ since 2008 my opinion of this country may be skewed.... but it's not just me who thinks it's tip top. New Zealand regularly ranks highly in the various lists of 'Best Countries in the World" and such-like, including winning "World's Best Country" four years in a row in the Telegraph Travel Awards. I think it's easy to see why. It's a relatively safe, westernised country, with arguably some of the best scenery in the world. It's easy to travel around with decent roading infrastructure and reliable public transport options.... But what does it for me is the sheer amount of open space (or lack of people.. depending on how you look at it).
Image courtesy of Ali Zifan - https://commons.wikimedia.org/w/index.php?curid=43475698
A country similar in size to the UK, yet it has 60 million less people... In saying that though, tourist numbers do mean that even in some of the remote parts of the South Island, you're never too far away from a fellow campervan'er!
Unless you're on a tight timescale there is no better way to see the country than in a campervan. With no fewer than 15,000 camping spots (2134 which are free of charge) you're never short of a place to pull up for the night. Often (especially the DoC sites) they are located in some of the most beautiful spots...
Assuming you already have a van, some good music and a full tank of fuel my only other recommendation is to keep that age old (if not cliched) adage in your mind...
"Take only photographs, leave only footprints"
or tyre tracks :/
"Wake up to a different view every day"
For more information on Campsites in New Zealand check out the following links
Department of Conservation: Campsites - http://www.doc.govt.nz/parks-and-recreation/things-to-do/camping/
WikiCamps NZ - http://wikicamps.co.nz/
Campermate - https://www.campermate.co.nz/map/index
100% Pure New Zealand - https://www.newzealand.com/int/holiday-parks/
Holiday Parks Association of NZ - https://www.holidayparks.co.nz/find-a-park