There are a few shops around Auckland that do it. Global, Dive Centre, A Dive Shop to name a few. Unsure on costs. Dive HQ Whangapraroa/HBC(?) do it for the same prices as air.
For your average Kiwi diver I dont see much advantages for using Nitrox. Most are doing relatively shallow and short scallop and cray dives and probably only one or two a day. NDL's are not really the limiting factor here.
Now its been way to long since I has been up to Tutukaka, but I know in the past getting Nitrox up there was a PITA if not impossible. This made no sense to me as the Knights are a perfect example of where using nitrox is advantageous.
Of course Northland Dive still do Nitrox and the diving up there also benefits well from Nitrox.