National MP Andrew Falloon quits politics after it was revealed he allegedly sent indecent images to more than one woman.

Yesterday it was revealed he apparently sent an indecent image - not of himself - to a 19-year-old woman.

Today he has written to leader Judith Collins and Speaker Trevor Mallard resigning, effective immediately.

Collins said she could no longer trust his story. She understands he sent pornographic images to more than one woman.

The Rangitata MP won't be standing again in his south Canterbury seat.

The National Party was notified on late Friday afternoon of an alcohol-related incident involving Falloon in which he behaved in a way "unbecoming of an MP".

Earlier today, Collins told Morning Report she thought Falloon should resign as she could no longer trust his story.

"I think he should resign from Parliament today, now that there are further statements and I can no longer trust his story. I believe that that would be the best thing for him ultimately. He is clearly now with his family and receiving professional assistance. It is I believe better for him, better for the young woman who is my first priority, and for Parliament, that he resigns.

"It was clear to me yesterday that he was admitting what was being alleged, what had been alleged by the young woman, and that I've now been advised by media that Andrew Falloon has now changed his story."

"It may well be a pattern of behaviour" - Judith Collins duration 10′ :43″ from Morning Report Add to playlistPlaylist Download Download as Ogg Download as MP3 Play Ogg in browser Play MP3 in browser "It may well be a pattern of behaviour" - Judith Collins

Collins said she had not seen the image in question. But she said it was of a pornographic nature and not of a male.

"My first priority yesterday was the young woman who is now dealing with the terrible trauma and I've reached out to her and made it very clear that if there's any assistance I can give I will do so."

Police investigated Falloon after receiving the report, but determined it did not meet the threshold for prosecution.

Collins said: "If they have any [information] and they wish to provide it to me it would be treated with the utmost confidence... I would treat this extremely seriously".

"I have no questions in my mind, no doubt that he did send that message," Collins said. "Any suggestion that he didn't is simply wrong."

He only knew the young woman in a professional sense, Collins said.

"It was clear to me yesterday that he was admitting what had been alleged.

"I am very concerned because I have today - it was very early this morning - received advice from media of other instances of this with at least another person.

"That indicates that it is unlikely to be a solitary incident.

"It may well be a pattern of behaviour - but the only thing I have to go on is the information from the media and I have not seen the evidence myself.

"Obviously he has, from what he has said to me, significant mental health issues, however I am very very aware of the young woman involved and any other women involved if this is a pattern of behaviour."

Collins said she did not have the ability to sack Falloon, but her advice was he should resign today.