While it does sound as if 3 weeks is nowhere near enough time, we did both islands in that time (admittedly some parts were not visited), but we did everything and more than we originally set out to - see my two earlier posts.
It's a brilliant country to visit, you'll enjoy every minute!
We only had 3 weeks and we hadn't planned our itinerary!
Most of our driving was done late afternoon into the night to get from one camping site to another - if you use Top 10 Holiday Parks get yourself one of their Members Cards for a few NZ$. It gives you discounts at their camp sites and...
My wife and I Honeymooned there in 2013 taking a camper van round both north and south islands.
Loved the south and have many personal highlights.
Kaikoura - Whale watching and swimming with Dolphins (freezing cold, but when you're out there with around 200 Dolphins it doesn't matter!)...