The most important factor working in favour of the survival of the monarchy in New Zealand is the general public's apathy towards it, and any proposals for change. In the afterglow of Prince William's tour, Dr Paul Moon, The Waikato Times, Chris Trotter and numerous other commentators have broadly expressed this sentiment as "why bother with a republic?". The other caveats are "the monarchy is popular" or "nothing will change". There's also the wider claim that there's no republic debate whatsoever (as Ali Ikram sort of claimed), something that also serves proponents of the status quo. This appears to be the strategy of Monarchy New Zealand - don't engage in the debate, claim no-one's interested, while vociferously trying to rebuild the monarchy's flagging, inactive support base.
On the surface, "why bother?" seems compelling. Questions such as: what's the
urgency? what's the point? what's the need? kill off any rational
discussion. The upside is that the argument "it ain't broke, don't fix it" has been sidelined from the public discourse - although Jim Hopkins and Garth George tried to make the claim, most people see through their thin rhetoric and red herrings. The downside is that it's a difficult to refute, because it's not an argument or a refined piece of rhetoric - it's simply an abrogation; a way of shutting down a discussion the proponent would rather not have.