I'll be off end of this month! 

I'll be there for 10 nights and will be using a 4 berth campervan from Kea. My itinerary will be as follows but it's just a guideline as we don't want to rush and might stay longer at certain places...

Christchurch - Timaru - Dunedin - Te Anau - Milford Sound - Queenstown - Lake Tekapo - Christchurch.

Campervan is widely available in NZ but the caveat is most (if not all) insist on a minimum rental of 7 days. So to the earlier posting who's planning only 5 nights, I think a rental car would be more suitable.  Please also be advised that there will be an influx of visitors during the Rugby World Cup in September, so rental cars and accommodations might be limited during that period. Summer of NZ at the end of the year is also peak season. Book early to avoid disappointment.

There are no highways at South Island and most are just 2 lane trunk roads. Speed limit is maximum 100km/h and don't expect to negotiate your way through if caught. Compulsory seatbelts for all passengers, NZ$150 instant fine if caught. For campervans, there is strictly no camping by the side of the road and freedom camping should only be done with prior permission if the spot you chosen is privately owned. There are numerous free camping sites with only basic facilities and these sites only allows "self sufficient" campervans. Other than that, there's other paid camp sites with charges ranging from NZ$30 - $70 for 2 persons. (powered sites for campervan)

I'll try to update on developments and perhaps also post when I am there.

Cheers!