I would maintain that the base is of strategic importance. Interested in other people thoughts.
I think a range of other considerations other than Defence come into play here. First up - competition to the Auckland Airport monopoly, (if) a government ever permitted domestic flights to/from Whenuapai to serve the rapidly growing North Shore population (precedent is RNZAF Woodbourne/Blenheim airport).
Second - high demand for new housing on a large stock of Crown land, following the very successful conversion of RNZAF Hobsonville. Of these two, I would say housing would be the higher political imperative.
In terms of Defence operations, the SAS base at Papakura is a
looong drive south from Whenuapai, would leasing helo space at Ardmore be a more practical option, or expanding a hangar at Papakura?
Other need is for facilities for the Seasprites. By their nature these do not require extensive runway facilities, so lending support to the 'conversion to housing' scenario.
More pertinently, Government has signalled an eventual move out by the recent decision to locate the P-8 Poseidon squadron at Ohakea. All too easy to imagine a similar move for the replacement Hercules (whenever that is announced).
Finally, I can't recall if there are important air bases located within the city limits of any other major international cities? (though I appreciate am flattering Auckland more than a bit to include in such a list)