Yeah as I said, anything to, from and within NZ is going to be in economy. So seems pointless to join up with Airpoints from what you described...
J class is only when heading to SE Asia, Pacific Islands or the USA.
What you describe actually makes me think NZ airpoints may be a good program for you - you earn the status from the SE Asia flights, and use the upgrades (2 per year) on your flights to NZ, on Air NZ.
J class travel to Asia & USA is particularly lucrative. MEL-SIN or MEL-BKK in J on either SQ or TG will earn $300 for return flights (or 37.5% of what is required for Gold). By contrast you would earn 9,375 SQ points - for SIN - (18.75% of that required for Gold). For a return to the USA it is either $520 and $560 for a return trip, depending on who you travel with - or 70% of getting to Gold).
Only trouble may be redemptions - depends on which class you are likely to book award flights on. THe burn rate for NZ airpoints for J class awards is not brilliant.
I went down this path based on 3 return flights to Asia (via SIN) per year in J class (actually I should say C class if talking Star Alliance).