It's one of the downsides of NZ operating a loyalty program which is based almost entirely off dollar for dollar "any seat awards" - they don't have to reserve award inventory because award inventory is the entire plane - for their own FFP members. R class (Airpoints upgrade inventory) is constrained to a minimum, X class (staralliance awards) even more so.

There are very limited "discounted business class awards" available to NZ *G and higher, but they require 2790 NZAPD for a one way to the US, plus taxes and fees - you actually get better value booking Y and upgrading. (And to make it worse, I believe these consume the same award inventory - they aren't R class)