Hi Werner,
This old thread might give you some ideas
Route planning
The ferry between Wellington and Picton costs about $300 for a mid-sized campervan. You can get a cheaper deal if you book online. And an even cheaper deal ifyou join the New Zealand Motor Caravan Association -
New Zealand Motor Caravan Association Inc. NZMCA membership also qualifies you for discounts at various camp sites and holiday parks.
Unfortunately, roadside camping (freedom camping) isn't allowed in certain parts of NZ unless your campervan has a Self Containment Certificate stating that it can last for two or three days without having to dump waste water or rubbish. Some of the hire companies have had their vans certified, but not all of them.
Even more unfortunately, you're not allowed to freedom camp in other parts of NZ at all. Some regional and local councils don't like it, so they have banned it. Other areas are environmentally sensitive, so it makes sense. The NZMCA publishes a travel guide for members that explain all the restrictions.
Fortunately, there are heaps of rest places and DOC sites where you are allowed to camp overnight. Facilities range from non-existant or pretty basic to decent toilets and shower blocks, although these ones usually have an honesty box and charge a small fee (less than $10 a night). You're also allowed to take advantage of the 'Queen's Chain' which is an ancient law that allows you to camp within 50 metres of Crown-owned rivers and streams. I'm sure you'll find some official guff about that if you search on Google NZ.......