Helen Clark has been confirmed as director of the UNDP by the UN General Assembly. She is expected to start by 20 April - just three weeks away. Which means there will be a by-election - and soon. Section 129 of the Electoral Act requires the Governor-General to issue a writ for a by-election within 21 days of a vacancy being declared; s139 states that the latest day a writ can be returned is 50 days after its issue. So, that's 71 days for the entire process, which means that if Clark resigns on the last possible day the by-election will have to be held and the results certified by 1 July.
An interesting question is whether the Electoral Commission has the money to do it. As their Briefing to the Incoming Minister [PDF] made clear, they are grossly underfunded; their budget has not been increased since 1994, and it has got to the stage where they can now no longer afford to fund basic election education (all those orange man ads). One of the things I'll be watching for in the upcoming Budget is whether the government corrects this, or whether it continues to let our basic electoral infrastructure rot in order to save a few dollars.
Correction: Whoops! The Electoral Commission has nothing to do with by-elections; they're handled by the Chief Electoral Office.