Andrew Falloon has quit politics after the Prime Minister's office was alerted to an incident involving the first-term National MP.
The National Party says he is dealing with "significant mental health issues" after a friend's death.
The Prime Minister's office has confirmed it was sent "a piece of correspondence" relating to an incident involving Falloon last week which was forwarded to National Party leader Judith Collins.
"With the correspondent's permission the letter was forwarded to the leader of the Opposition," Jacinda Ardern's office said today.
"We treat all correspondence confidentially. All matters to do with Andrew Falloon's resignation are a matter for the Leader of the Opposition."
The Herald understands the incident was not criminal but was deemed "inappropriate for an MP". It's also understood the incident is not related to an extra-marital affair or the leak scandal which engulfed the National Party last week.
Falloon, the MP for Rangitata, issued a statement this afternoon saying he will not contest September's election, citing concern for his "own health and wellbeing". He said he had made a "number of mistakes".
Collins said Falloon was facing "significant mental health issues".
Falloon becomes the third MP to quit after Collins was appointed leader last week.
Senior MPs Nikki Kaye and Amy Adams have decided to quit politics, but said Collins' promotion to the leadership was not a factor in their decisions.
Falloon, a first-term MP, said that he told Collins today that he would not contest the September election.
"As I noted in my maiden speech three years ago, when I was younger I lost three close friends to suicide," he said in a statement.
"It was an extremely difficult period in my life. Unfortunately, recently, another friend took their own life, which has brought back much unresolved grief.
"I have made a number of mistakes and I apologise to those who have been affected.
"Recent events have compounded that situation and reminded me of the need to maintain
my own health and wellbeing. I have again been receiving counselling.
"I want to thank Judith for her support during this time and I look forward to helping a new candidate in the Rangitata electorate in any way I can.
"I apologise for this disruption to my colleagues and to those I serve in mid- and south Canterbury."
Collins said "the National Party was advised of an issue relating to Andrew late on Friday afternoon and we have dealt with it this morning".
"Andrew is suffering from significant mental health issues and his privacy, and that of his family, must be respected."
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• CASPER Suicide Prevention
If it is an emergency and you feel like you or someone else is at risk, call 111.