I wonder if the proximity of the other 3 other airports to the north of Auckland (15-25NM) - i.e. (Woodbourne (NZWB), Hobsonville (NZHB), North Shore (NZNE)) , has something to do with this, i.e. that planes leaving Auckland are steering clear of the area....
That and the fact that NZAA is pretty much an east/west runway.