Visitors from Japan don't need a visa if they are staying 3 months or less. After that you can
apply to extend up to 9 months or maybe 12 months in total. Then you would have to stay out of NZ for 12 months before you could come in again.
I think maybe INZ would only extend your visa 3 months at a time or at least that's what they do with extensions to VV holders from the UK, but they have a 6 month visa when they first enter NZ, so they get an extension for another 3 months and, if circumstances are important enough, they can get a final 3 months making 12 months altogether. I can't find anything that says whether you would only get another 3 months + 3 months later on or a whole 6 months if you applied for an extension.
Whatever, you only get to stay a maximum of 12 months in NZ, the last 3 months being granted if there are special circumstances.