Spot on.
Quite apart from LPS500's entirely reasonable points, the real impediment is
politics.
The people that would be most upset by additional air movement noise would be those that live in the elctorates of Helensville and North Shore. These are both ultra-safe National seats and are currently held by The Rt. Hon John Key (Prime Minister, Minister of National Security and Intelligence, and Minister of Tourism) and The Hon. Maggie Barry (Minister for Arts, Culture & Heritage, Minister of Conservation, and Minister for Senior Citizens). There is no way in the world that civilian RPT use of Whenuapai is going to occur under a National Party lead government.
Anyone who wants dual use of NZWP better get busy working for a change in government, preferably while leaving Helensville and North Shore as nice safe opposition-held seats.