While in China I had an interesting conversation with a friend of mine, in which we discussed what the constitution of New Zealand would look like if we ever became a republic. A large issue with republicanism in New Zealand is that the highest authority in a traditional republic, who reserves ultimate approval for all new legislation, is an elected president. I have to question the merits of placing one elected figure in such a position of power in a country as small as New Zealand. At present, the Queen's approval on legislation is little more than a token nod to her position as head of state, with all true legislative power residing in the elected, multi-party parliament. The whole purpose of MMP in New Zealand is to allow more power to smaller parties and prevent a monopoly on political power by one ideological group (Labour or National), so why would we rest more power in the hands of an executive president?
My answer was to not involve a directly elected head of state. The executive government should be formed just as it is now, out of a coalition of elected parties in parliament with the Prime Minister as head of state. This of course violates the principle of separate legislative and executive power that modern democracy clings to (or professes to cling to) so dearly, so who should hold a check on the law-making ability of the government?
I would hold the Supreme Court up as the ultimate authority on all new legislation. No bill should become law without an investigation and ruling on its constitutional validity, with a majority of Supreme Court judges favouring the bill. The Supreme Court could also be tasked with retrospectively examining already-passed acts of parliament to ensure that they too follow the guidelines of New Zealand's various constitutional documents.
An issue that Matt raised was the appointment of the judges; if they are directed elected by the people or appointed by the government then their appointment becomes less about their legislative ability and more on their political interpretation, as we see with the American Supreme Court. However, this aside it is still a very interesting model of a republican New Zealand that holds Rule of Law as its highest political ideal.