Air New Zealand is trying to cut costs where ever possible at the moment. Kicking out some more routes, getting rid of the B1900Ds, retiring a 767 and they are about to retire some of the ATR72-500s... It is clear that they are feeling the pinch that Qantas Group is putting against Air New Zealand.
Much of what you outline are decisions that well and truely predate JQ arriving in the regional market. As for retiring the 500s, you make it sound like there is a scaling back of operations. I'd hardly say spending big on one for one replacements (plus extra growth aircraft) is even in the same category.