The Port Waikato electorate by-election date has been announced this afternoon by the prime minister.
Chris Hipkins said the by-election would be held on Saturday 25 November, which he said was the earliest possible date.
The death of the Act Party candidate for Port Waikato, Neil Christensen, announced on Monday, means the by-election is required, according to rules in the Electoral Act 1993.
The act requires that the current election for that seat be cancelled, and a new by-election be organised as soon as possible.
"Following consultation with other political parties I've confirmed November 25th as the by-election date," Hipkins said.
"Everyone was of the view it should occur at the earliest possible time."
Announcing the by-election date on Tuesday afternoon, Hipkins said the situation was "a technicality", but "the process has to be fair".
He said if one of the main candidates had died it could have had a considerable impact on the election, so the Electoral Commission must follow due process.
However, after the election there may need to be a review to see if a fairer system is needed, he said.
The process required by the Electoral Act if a candidate dies during an election meant there would be an extra list MP as a result, but it would not be known until the votes are counted which party would benefit, Hipkins said.
Port Waikato electorate voters who want to vote for their preferred party must still go ahead as before, and cast their party vote according to the dates for the national election.
That means that for the electorate, voting for both a party and an electorate MP will require voting on two different dates.
Hipkins acknowledged that, and said it was not an ideal situation.
Labour had not decided whether it would stand a candidate.
Having a byelection was the right thing to do in the circumstances, ACT leader David Seymour said at a media briefing in Christchurch on Tuesday.
The law existed to ensure a candidate's death would not affect the outcome of an election which was important, he said.
"I think that's respectful and right and proper not to carry on and have the election in spite of his passing [referring to the death of Neil Christensen] but actually have a fresh election in the Port Waikato electorate ... it's the right thing to do."
People's choices should not be pre-empted and he was comfortable with how the law was being applied.
There were some inconsistencies bolting on an MMP system to the old first past the post system, Seymour said.
Considering how few times the situation was likely to arise, he was confident it was the right thing to do.
Other important dates for the Port Waikato by-election
Hipkins said the by-election Writ Day (a formality where the Governor General issues formal direction to the Electoral Commission to hold the by-election), would be Monday 16 October. (This timing can affect special votes).
The deadline for candidate nominations to be received is midday, Friday 20 October.
And the last day for the return of the Writ (the announcement of the official results) is Friday 15 December.