NZWB may be a civil/military airfield, but the only thing they share is the runway and some of the apron.
NZWP would require significant infrastructure works to separate the civil world from the military world and the government is not going to fund that for a regional airline start-up with a chequered history in the management ranks, and I doubt KRA will have anywhere enough capital to build it themselves.
Where do you think you'll put a terminal, decent sized carpark "close" to said terminal and new roading to aforementioned terminal/carpark??
Too many $$$, not enough ROI.