I'm not at all sure about this. C, D and Z are the ordinary three NZ business fare-buckets. Expertflyer only quotes C D and Z fares for NZ flights, nothing for J, although it does show availability in the J bucket.
The "special" discounted LHR-AKL business fare I bought off the AirNZ website 10 months before travelling booked into Z.
I would suggest a PM to Kiwi Flyer who I am sure will clarify matters.