Iain Lees-Galloway has spoken out after being sacked as a minister over an inappropriate relationship with a staff member in one of the departments he headed.
Prime Minister Jacinda Ardern said the allegation related to an "inappropriate relationship" with a former staffer that had spanned a year.
His political career is now over and he will not contest September's election.
Lees-Galloway - who is married with three children - said in a statement that he had "acted completely inappropriately in my position and can not continue as a minister".
"I accept the Prime Minister's decision and apologise absolutely," he said.
"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."
Lees-Galloway, 43, had told Ardern it was a consensual relationship that had ended several months ago.
Ardern said she had lost confidence in Lees-Galloway as he had improperly used his position of power.
"In undertaking this relationship he has opened himself up to accusations of improperly using his office," she said.
"He has not modelled the behaviour I expect as a minister that is in charge of setting a standard and culture in work places.
"His actions have led me to lose my confidence in him as a minister."
The Prime Minister did not want to identify the woman involved, but added that she wasn't an intern.
"I'm advised at the time the relationship commenced, the woman was not in his office but was in one of his agencies."
She drew a distinction between moral judgments and judgments about the proper conduct of a minister, in this case the Minister for Workplace Relations.
Whether Lees-Galloway had other affairs was immaterial, she said, given that this single affair was enough to sack him from Cabinet.
Position as minister 'untenable'
Ardern called Lees-Galloway into her office at 5.45pm yesterday, when he admitted the relationship with a staffer in his office who was based in one of his agencies.
"The minister has shown a lack of judgment over a period of 12 months," Ardern said.
This made his position as a minister "untenable".
The issues were the effect of the lack of judgement on the culture and relationships in the workplace, given Lees-Galloway's ministerial portfolios, Ardern said.
Asked about whether she was aware of rumours about Lees-Galloway, she said she didn't want to act on rumours and had acted on an allegation that had been confirmed.
"We all have a role to make sure we maintain standards in this environment," she said when asked whether this showed poor behaviour among politicians.
She said she would have acted in the same way regardless of how the information had come to her.
Asked about whether it was now open season on MPs' affairs, she said it was the inappropriate use of his ministerial office that was the compelling factor in this case, and also the fact he held the Workplace and Safety portfolio.
Ardern wouldn't be drawn on whether affairs were all a matter of conduct unbecoming, saying that she will deal with the particular circumstances, which is what she had done in this case.
"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."
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.