National MP Andrew Falloon's explanation for the sex text scandal that ended his political career is understood to be that acquaintances at a party sent the offensive message.

Falloon, a first-term MP, resigned today and it emerged that a pornographic image had been sent to a young woman.

The Herald understands that Falloon's version of events is that he was at a party several weeks ago and briefly left his phone unattended - and at that time acquaintances used it to send the sexual image in question.

It is also understood that police have already investigated the incident and no action was taken.

New National leader Judith Collins was alerted to the issue after the parents of the young woman, a university student, contacted the Prime Minister's office.

Jacinda Ardern confirmed today that her chief of staff passed on this information to Collins, with the parents' permission.

Despite insisting he did not personally send the message, Falloon is believed to have offered his resignation to Collins today and she accepted.

Police conducted an investigation after receiving a report of an individual sending an unsolicited image, a spokesperson said.

"The investigation determined it did not meet the threshold for prosecution. As such, no further action will be taken."

Earlier today, Falloon, the MP in the Rangitata seat, said he had made a "number of mistakes".

• MP who quit reportedly sent inappropriate image to young woman
• Barry Soper: Collins had a chance to act swiftly with Falloon scandal - she didn't

In his statement, the 37-year-old said: "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.

Collins said in her statement Falloon was facing "significant mental health issues".

Sending harmful content is one of Collin's zero tolerance issues.

As Minister of Justice, Collins put up the Harmful Digital Communications legislation which primarily targeted cyber-bullying, and covered the sending of objectionable material.

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."

Prime Minister Jacinda Ardern said the matter was for the National Party to deal with and they had dealt with it appropriately by asking for permission to forward the information and by keeping it confidential.