Prime Minister Jacinda Ardern has sacked Iain Lees-Galloway as a minister over an inappropriate relationship he had with a staff member in one of the departments he headed.
The relationship lasted 12 months and has cost Lees-Galloway his political career - he will not contest September's election.
Lees-Galloway, 41, is married with three children. He said he had "acted completely inappropriately in my position and cannot continue as a minister".
Ardern said ministers having affairs was not necessarily inappropriate behaviour, and it depended on each individual circumstance.
But Lees-Galloway's position was untenable because he was the Minister for Workplace Relations and Safety, and he had opened himself up to accusations of improperly using his power.
"He has not modelled the behaviour I expect as a minister in charge of setting a standard and culture in workplaces," Ardern said.
The Prime Minister did not want to identify the woman involved, but added that she wasn't an intern.
Asked about whether it was now open season on MPs' affairs, she said the compelling factor was the perception of the inappropriate use of his ministerial position, in particular the Workplace and Safety portfolio.
Ardern wouldn't be drawn on whether affairs were automatically conduct unbecoming of a minister, saying that she would deal with each particular circumstance.
"At any point where I have things that concern me, I deal with them in an appropriate way, but I don't want to be drawn into hypotheticals or rumours without substance."
Whether Lees-Galloway had other affairs was immaterial, she said.
Position as minister 'untenable'
A person had contacted National leader Judith Collins yesterday about Lees-Galloway, and Collins had passed the information on to Ardern after Question Time.
Ardern called Lees-Galloway into her office at 5.45pm yesterday. He admitted the relationship. He told Ardern it was consensual and had ended several months ago.
"The minister has shown a lack of judgment over a period of 12 months," Ardern said.
Ardern said this had happened in a highly charged political environment, but Lees-Galloway's behaviour was out of line for a sustained period of time.
She had been advised that taxpayers' money had not been inappropriately used by the pair, but she would ask Ministerial Services to ensure that was the case.
Lees-Galloway released a statement saying he would not stand in the election:
"I accept the Prime Minister's decision and apologise absolutely. I have acted completely inappropriately in my position and can not continue as a Minister.
"I have apologised to my family for letting them down. Please appreciate their privacy.
"I also apologise to anyone who has been hurt by my actions."
He said no further comment will be made today.
He has deleted his Twitter account and his Labour Facebook page.
The father-of-three has been in Parliament since 2008.
Ardern's impromptu 11am press conference
Ardern called an unscheduled 11am press conference after Collins revealed she had forwarded information about alleged "inappropriate behaviour" by a Labour minister to her.
Ardern said the information did not come from the woman in question, but from a third party, whom Ardern did not know personally.
The third party had sent the information without the woman's knowledge, Ardern said.
Asked whether she would ask her other ministers about similar behaviour, Ardern said there was a "constant expectation" around ministers' behaviour.
"We all have a role to make sure we maintain standards in this environment."
Ardern said Collins had handled the situation differently to how she had handled the tip-off about National MP Andrew Falloon, but that was up to Collins.
Lees-Galloway is the MP for Palmerston North and was Minister of Workplace Relations and Safety, Immigration and ACC.
Carmel Sepuloni will become the Minister for ACC, Andrew Little will become the Minister for Workplace Relations and Safety and Kris Faafoi will become Minister for Immigration.
Previous botched handling of Sroubek case
Lees-Galloway has previously been in hot water over his botched handling of Czech drug-smuggler Karel Sroubek's immigration case.
The minister stood by his original decision to grant Sroubek residency even after a review concluded there were "powerful" reasons to deport him.
The review, by Mike Heron QC and released in October last year, was ordered after Lees-Galloway's controversial decision to grant Sroubek residency despite a conviction for drug-smuggling and admitting he used a false identity to gain a resident visa.
Lees-Galloway revisited the case after it emerged that Sroubek may have travelled back to the Czech Republic, and eventually issued a new deportation notice to Sroubek, who remains in a New Zealand prison.
The Heron review found that the Immigration NZ processes were adequate but could be improved.
It said that ministers applying absolute discretion may have limited time and did not usually receive free and frank advice on deportation cases - though ministers were also free to take more time and seek further information.
"It is obvious to state that a process which allows a minister to make a quick decision on a complex case with as little as an oral briefing and no advice is fraught with risk," the review said.
The risk could be mitigated if more decision-making was delegated to experienced experts, which would keep the minister "above the fray".
Collins passed information to PM
Collins said she passed the information she had received on to Ardern and that it was up to her to deal with it.
She said the allegation was serious enough that if it were about a National MP, she would seek more information.
Collins' apparent tip-off came to light when she was speaking about former National MP Andrew Falloon, who quit Parliament on Tuesday after it was alleged he sent inappropriate messages to a young woman.
Asked by MediaWorks if she had "received anything about Labour ministers or Labour MPs", Collins replied: "I have actually".
"I have advised the Prime Minister and I have asked for anybody who has that information to send it directly to her," she said.
"I spoke to the Prime Minister yesterday as we were coming out of Question Time. I asked to speak to her and I said I had received such a tip-off and I did not want to receive any information on it. I would be asking the person to send it directly to her.
"She has provided me with an email address for that and that has been passed on to the person who contacted me. I don't want to engage in this."
Cabinet Minister Chris Hipkins told RNZ Collins' comments were a matter for the Prime Minister to comment on but added that it seemed to be a contradiction for Collins to say she didn't want to get involved and then to put the matter into the public domain.
Collins disputed that she had thrown it into the public domain, saying on RNZ that she had been asked a direct question on the AM Show and she had answered it.
She said she had received an email from a member of the public yesterday and that led to her conversation with Ardern.