The Holden Republic has more in our exchange on constitutional issues. He's right; we don't differ greatly in our opinions. One difference is that he seems to have read my coment that things could simply be left to evolve as a "should". That's not really what I intended. It's not that we should simply leave constitutional structures beyond human rights protections to evolve as that we can, because there's less need for limits and the flexibility may be advantageous.
There's also a strong measure of pluralism about government structure here. To my mind the form of a government isn't nearly as important as what it does. What seperates good governments from bad and acceptable forms from unacceptable ones is the extent to which they support human rights, individual freedom (for everyone, not just the rich), and true equality of opportunity; are responsive to their people; and have proper checks and balances to prevent abuses. There is no one best way to do this - modern liberal democracy does it well, constitutional monarchy seems to do OK, and concievably even a benevolent dictatorship might be able to manage. Generally, our current structure is within the bounds of acceptability. It has its flaws - too much executive power, too few protections for human rights - but they require tweaks rather than fundamental change. We don't need to move to a republic to address those concerns, hence the slow drift and republicanism-by-a-thousand cuts rather than a clean break.
(That said, I also think that monarchy is incompatible with human dignity. We are citizens, not subjects; we rule ourselves, rather than needing to be ruled by another. While constitutional monarchy (backed by the memory of what happened to Charles I and James II) has removed the arbitrary excesses of kingship, it can't erase this. But it's hardly worth building a guillotine over.)
As for ultimate republican forms, others have talked about codifying the reserve powers, twinking out the governor-general and splicing in a ceremonial president in their place. This would be the easiest path to a republic, but poses some danger of the president exercising their powers for political reasons (think of Australia's constitutional crisis here). At present we have strong conventions against such abuses, but these could easily break down in the changeover.
A more interesting option is mentioned by Brian Easton in the final pages of The Whimpering of the State. He proposes a "Swedish-style" republic, where
- the constitutional law would state the lawful successor of Queen Victoria would be the head of state of New Zealand (thus respecting the Tiriti o Waitangi as the foundation of the New Zealand constitution). There would be no other reference to the Head of State (as there is none in the Swedish constitution);
- the duties of the Governor-General or Queen's representative, appointing the Prime Minister and related activities, would be taken over by the Speaker of the House acting in partnership with two deputy speakers. This Swedish solution emphasizes that the power of governance comes out of parliament.
- a bill of parliament would become law when the relevant minister signs it and the Chief Justice is advised.
(The Speaker and deputies would be elected by a supermajority, and a dissolution would require the agreement of all three, thus providing balance and reducing abuses.)
This would require greater change than the first option, but I think the result would be better. It fits well with the tone set by MMP and moves us more towards consensus politics. It builds on and amplifies existing conventions regarding the Speaker and their neutrality, and the requirement for a supermajority would enhance this further. And it saves us from having to debate the role of a president (which has been problematic in Australia). Unlike Easton, however, I don't really see the need for a "figurehead clause". Napoleon's advice applies: if we're going to do away with the monarchy, then let's do away with the monarchy, and not leave it lying around where people might still continue to believe in it.