Hi Zivel,
I had a similar idea the other day, looks like you beat me to it

. This is a country that really deserves a CC map.
Anyway, more to the point:
As Andy said, I think it will work well as a smaller map. To deal with linearity, you could use airports (maybe Auckland City, Wellington City, Christchurch, Invercargill, etc.) that would all be connected. I know you mentioned you're thinking of simple gameplay, and I prefer simple maps to ones with autodeploys, bombardments, etc., but I feel that it would be too much of a one-front war otherwise. Alternately, you could use SE AUS (say, from Melbourne up to Brisbane) and turn it into more of a triangle (SE AUS, NZ North Island, NZ South Island), but I assume you want to make it just NZ. As Ender516 mentioned, the physical geography would really help (i.e., the South Alps to seperate South Island into west and east).
Best of luck,
nosorozec39