54 Term of office of member of Parliament (1) Where an election is held for any electoral district, the person whose name is endorsed on the writ issued for the election as the person declared to be elected shall, subject to this Act,— (a) come into office as the member of Parliament for that electoral district on the day after the day of the return of that writ; and(b) vacate that office at the close of polling day at the next general election. (2) Where any person whose name is entered on a party list submitted pursuant to section 127, is declared by the Chief Electoral Officer to be elected as a member of Parliament, the person shall, subject to this Act, (a) come into office on the date after the date of the return made by the Chief Electoral Officer pursuant to section 193; and (b) vacate that office at the close of polling day at the next general election.
MPs reign until the close of polling. But new electorate MPs don't come into office until the day after the writ (the formal advice to Parliament of the outcome of the election by the Chief Electoral Officer) is returned under section 285 of the Electoral Act 1993. That's not scheduled to take place until 27 November (Chief Electoral Officer: "Election date announcement puts Chief Electoral Office in top gear"). And new list MPs don't come into office until the following day, after the Chief Electoral Officer has declared those elected pursuant to the list. The other consequence of the quirk is that our constitution needs to provide for the temporary tenure of ministers who are not MPs. The most basic feature of our system is that one needs to be an MP in order to be a minister. (In principle, it's not entirely incontrovertible, as some have suggested we could allow non-MP appointments of ministers. But that would require a constitutional change.) The MP-lacuna during the electoral transition period would ordinarily undermine that feature. But the answer - and smooth constitutional transition - is found in section 6(2) of the Constitution Act 1986:
6 Ministers of Crown to be members of Parliament (1) A person may be appointed and may hold office as a member of the Executive Council or as a Minister of the Crown only if that person is a member of Parliament. (2) Notwithstanding subsection (1) of this section,— (a) A person who is not a member of Parliament may be appointed and may hold office as a member of the Executive Council or as a Minister of the Crown if that person was a candidate for election at the general election of members of the House of Representatives held immediately preceding that person's appointment as a member of the Executive Council or as a Minister of the Crown but shall vacate office at the expiration of the period of 40 days beginning with the date of the appointment unless, within that period, that person becomes a member of Parliament; and (b) Where a person who holds office both as a member of Parliament and as a member of the Executive Council or as a Minister of the Crown ceases to be a member of Parliament, that person may continue to hold office as a member of the Executive Council or as a Minister of the Crown until the expiration of the 28th day after the day on which that person ceases to be a member of Parliament.
Existing ministers can continue as ministers for 28 days after election day. People can be appointed ministers and continue to hold tenure for up to 40 days, if they were a candidate in the election but have not yet formally come into office.
In previous government transitions under MMP, the determination of Prime Minister and new ministerial positions was settled after MPs came into office. However, with the rush to get John Key to APEC, these arrangements are being expedited and he will be appointed before then - perhaps even on the weekend. In a formal sense, that means he will be representing New Zealand as the most senior member of our Executive, but not yet as a member of our Parliament!