Labour MP Winnie Laban accepted a job at Victoria University before she told her party leader and colleagues of plans to leave politics by the end of the year.
The MP for Mana said she saw an advertisement in the New Zealand Herald for the Assistant Vice-Chancellor Pasifika at Victoria University about two months ago, applied, and was accepted three weeks ago.
She told Labour leader Phil Goff about two weeks ago. Other Labour MPs and party president Andrew Little were told yesterday.
"I unilaterally did this. I saw an opportunity," Ms Laban said.
"Phil would have liked me to stay on, but he was also supportive of what I wanted, and I was pretty determined about what I wanted."
Mr Goff said that although Ms Laban had accepted the position, it was still conditional on his approval.
Ms Laban is expected to stand down in October.
A byelection, probably in November, will cost about $500,000, with a campaign spending limit of $40,000 for each candidate.
Labour is likely to bring in a candidate from outside its parliamentary caucus to further renew the party, but also to keep Judith Tizard, who is next on Labour's list, from returning to Parliament.
Mr Little said it was too early to speculate on who might stand, but hinted that a Pacific person would be ideal.
"Obviously we want someone who is a good fit for that electorate, given it has a pretty diverse demographic profile. The search for that will begin pretty much straight away but there have been no names coming forward."
A spokesman for Prime Minister John Key said it was up to the local party organisation to select a candidate, though list MP Hekia Parata, who ran in Mana last election, has said she will seek nomination.
Ms Laban said she was not jumping from a sinking ship.
"I'm making a career move, and I've never seen Labour as being a sinking ship, but a quality party. I'm very happy with the leader's performance."
Mr Goff expected the byelection to be close.
He said Ms Laban's new job was not something she could have delayed until the next election, nor could she have done it at the same time as being an MP.
"We didn't want to stand in her way to take up a position that will be really important to the future of Pasifika education.
"We're naturally disappointed to lose her from caucus, but it does give us a chance for renewal for the Labour Party."
He said list MPs could stand for Mana, "but I haven't seen anybody put their hand up".
Ms Laban, who overcame breast cancer before the last election, is a popular member of the caucus.
She was the first Pacific woman MP, and has been in Parliament since 1999, and MP for Mana since 2002.
Mana has been a Labour seat since World War II.
She won in 2008 with a majority of 6155, but Labour's lead over National in the party vote was only 2508.
It will be the second byelection this term after Labour's David Shearer won in Mt Albert following Helen Clark's departure.
There is also the possibility of byelections in Te Atatu, where Chris Carter has been suspended from the Labour caucus, and Manurewa, where MP George Hawkins is seeking a seat on the Auckland Super City council.
* Mana MP Winnie Laban will quit Parliament by the end of the year to take up a position at Victoria University.
* It will force a byelection, probably in November, which will cost about $500,000.
* Ms Laban won in 2008 with a 6155 majority, but Labour was ahead by only 2508 votes in the party vote.