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 the 37-year-old 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.
RNZ understands the party was notified on late Friday afternoon of an alcohol-related incident.
A source confirmed to RNZ Falloon was said to have sent indecent pictures to a third party, who is not his wife, on at least one occasion.
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.