I’d recommend 1 night in Auckland, then 3 nights on the North Island, and then a ferry to the South Island. If you can spend a week there, and return the camper van or car to Christchurch for a flight to Auckland and home, it would be perfect. As long as you can spend at least 4 nights on the South Island, it will be worth it. But if your schedule or budget don’t allow that much, you might just stay on the North Island. The most interesting place on the South Island is Queenstown, but there is plenty to see all over.
Another consideration is that Thailand is generally easier to get to with far more flights that are often cheaper, and Thailand also has FAR more beach options because of its long southern coastline and especially the islands. The posh beach areas on Phuket are typically the most expensive beach hotels in Thailand, but there are other more remote beaches in Phuket with lower prices so it’s worth a look. Aside from Phuket you should also look at Ko Samui, Ko Pha Ngan, and Ko Phi Phi. You might also look at the town of Pattaya, which is the closest beach resort to the Bangkok Airport. Parts of town have a lot of single foreign men in certain bars at night, but most of the town is actually quite family friendly and very good value. Let me know if you have other questions. -Roger