There are restrictions on importing left hand drive vehicles into New Zealand. It is unlikely that you would be able to purchase a new caravan overseas and successfully import it. From Importing a left-hand drive vehicle (Factsheet 12):
Are there restrictions on LHD vehicles?
Yes. Most left-hand drive vehicles have to be converted to right-hand drive (RHD) before they can be driven on New Zealand roads.
For modern vehicles, this is either very expensive or impractical – especially if the vehicle has a modern frontal impact protection system (eg airbags and crumple zones).
The only vehicles that can be registered as LHD and driven on New Zealand roads are those that belong to one of the specific categories of LHD that are permitted by law.
Before going to the expense of importing a LHD vehicle, you should check very carefully that your vehicle is either belongs to one of the permitted categories or is capable of being converted to RHD. Otherwise, you may find it impossible to use the vehicle on New Zealand roads.
Additionally, New Zealand Customs has an information page on Motor vehicles, boats & aircraft. There are concessions on taxes for residents importing certain vehicles, which would imply that non-residents may still be permitted to import such vehicles but are not eligible for the concessions. Customs Fact Sheet 29, Advice on Private Motor Vehicle Imports indicates that motorhomes attract both 15% GST plus an additional 10% duty.
If you intend to take your vehicle with you when you leave, Customs states:
Temporary entry – tourist motor vehicles
If you are a tourist you may import a motor vehicle into New Zealand without payment of Customs charges, provided your intention is to take the vehicle out of New Zealand at the conclusion of your visit.
However, even in this case you would still be required to properly register the vehicle with NZTA for use on New Zealand roads.
You would almost certainly be better off buying a (used?) caravan in New Zealand and selling it when you are done. This will likely be considerably less expensive and time-consuming than attempting to import a vehicle. (Even if you were to successfully import a vehicle, you may not be able to use it for weeks or months as it passes through customs, biosecurity, licensing paperwork, and so on.)