Not PC's excellent post reminding us that government needs to be restrained by a constitution reminds me also about my wariness of the republican movement and those who talk simply about such things. Arguing about a republic and a written constitution is a bit more than simply saying it's a good idea - you have to know what the purpose of the constitution is for.
Simply codifying convention would be virtually meaningless. A constitution is a statement about the role of the state and the limits of the role of the state, and what freedoms and rights the state guarantees to its citizens. In New Zealand there are a handful of wildly disparate views of this. Think of those who, like followers of a faith, want to include the Treaty of Waitangi, assuming somehow that this will do something about the state other than accentuate ethnic division. Others will see it as a chance to enshrine not rights against people doing things to other people, but in favour of the state doing things for people. Put the Greens, Maori Party, Labour, NZ First, National and Libertarianz in the same room and you'll get very different answers - most fundamentally conflicting.
You see, this is not something that should be left to politically appointed commissions or groups of nodding heads from people who follow the same vision. It needs to be evolutionary, and indeed until politicians start respecting property rights and individual freedoms more widely, a constitution would merely entrench nanny state.
So for now, the matter of shackling the state should be a point of debate and discussion. I wouldn't give anyone on the left a chance to ever start creating a constitution worth wiping my arse on. A constitution should be about two things: What the state can't take do, and the creation, maintenance and checks and balances on that state. Unfortunately, with a Prime Minister who declares "the state is sovereign", and around five political parties happy to keep her in power, one way or another, precious little evidence of the former exists in the minds of the majority of existing MPs.