Presumably such paid memberships are a money maker, and I suspect AirNZ would be less concerned about crowding and wotnot provided the cash rolls in.

This is where the various models used by other airlines to separate various pax groupings (they do it differently too) might be useful. For example QF (Qantas Club & Business Lounges), SQ (KrisFlyer Gold and Silver Kris lounges), LH (Senator/*G and Business Lounges).

Of course the investment needed for two lounges is too much….