National MP Jami-Lee Ross is standing down from his portfolios and from the front bench of the opposition due to personal health issues.
In a statement Mr Ross said he had asked for time off on medical leave for "a few months".
"There are times in life where you have to put your own health and family first. As a husband and a father I need to do that at this time."
The Botany MP said he looked forward to returning to Parliament in the future to re-join the National caucus.
Mr Ross said the party's leader Simon Bridges had been "very understanding" in granting his request.
Mr Bridges told reporters in Auckland the issues were of a "deeply personal and private" nature and were revealed to him last week.
"I've got to be honest with you, this came out of the blue. You think you know your colleagues very well, but you don't always know everything that's going on for what are personal, sensitive, perhaps embarrassing issues."
He went on to say that perhaps "embarrassing" isn't the right word.
"I suppose I've said it best when I've said this is a personal and private health issue, actually it's not really for me to characterise it. But what I know from talking to Jami-Lee Ross is he's serious about his wellbeing, about dealing with that, and that's going to take him getting some help."
He said he would not be making any further comment on the nature of the health issues, but did confirm the decision was unrelated to the leak investigation.
National launched an inquiry into the leak in August after the Speaker Trevor Mallard cancelled a Parliamentary inquiry into the matter. That came after the leaker sent an anonymous text to Mr Bridges claiming to be a National MP and suffering from mental health problems.
Mr Bridges said he took Mr Ross at his word that he was not the leaker.
"Be really clear, that is speculation. I think you have to be really careful actually about what is a serious allegation in relation to someone where there is simply no basis - that I have - to say it is him, or anyone else in my caucus. It's a very separate matter from his personal and wellbeing issues.
"The reality is we've got a couple of things happening here. One, is Jami-Lee Ross and some personal and some private health issues. They're serious, you know he needs to take the time, a few months actually, to focus on that, his wellbeing and his young family," Mr Bridges said.
"And what we've also got of course is a leak investigation underway. We don't know who is responsible for that, and that's just a matter of working its way through."
National deputy leader Paula Bennett said the caucus was taking these concerns very seriously and treating them accordingly.
"We ask the same of the media and we ask the same of the public, that we get behind someone who is going through some pretty traumatic and hard times and we actually back them. And we don't do speculation that's unhelpful, unnecessary and untrue."
National MP Nikki Kaye, who took medical leave in 2016 when she had breast cancer, said her thoughts are with Jami-Lee Ross and his family.
Ms Kaye said she was well supported by the caucus, and people in general, when she took time out from public office - and she hopes Mr Ross will be too.
She said everyone's different but she hopes people will be respectful at a sensitive time.
Mr Ross' Transport portfolio will be picked up by Paul Goldsmith and Judith Collins will take over his Infrastructure portfolio.
Andrew Bayly will take over Mr Goldsmith's Revenue portfolio.