I am surprised to see that number, they must cost a fortune to import. Then there is the price of gas and parts, it would be a wealthy man's hobby.
Yeah, they aren't cheap, especially a nicely restored classic. Gas isn't such an issue as few of them are daily drivers.
The main problem is that steering wheel on the "wrong" side thing.
If you simply import a car from the US, it needs to be converted to RHD before it's road legal here. This can be done, but it's not cheap. The other way is to prove you owned and drove the car for x months overseas, ie, it's YOUR personal vehicle. Then it gets issued with a LHD permit and can be registered and driven. The permit stays with the car, and you are free to sell it later, with permit included.
So the usual way they end up here is either Kiwis working contracts in the US, or Americans doing jobs in NZ. Kiwis buy a car over there, drive it around, and bring it home. Americans bring a car with them, and sell it before they leave. Getting the US car shipped here is relatively straight forward, it goes in a shipping container, and other personal items get packed in with it. 5 weeks later it turns up at a port in NZ and you get it inspected, certified and registered.
About 1/2 the cars imported into NZ are already used, generally from Japan, so the import process is pretty streamlined and most regular production cars are allowed in.