Falloon's resignation followed an announcement on Monday morning that he would be retiring at the September election, citing mental health reasons and "unresolved grief" following the suicide of a close friend.
"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."
He also said he had "made a number of mistakes and I apologise to those who have been affected".
Soon after his statement, it was revealed a pornographic image was sent from his phone, to a young woman.
The recipient was a young woman attending university and her parents alerted the Prime Minister's office to the incident.
Jacinda Ardern's chief of staff, with permission, notified National Party leader Judith Collins on Friday.
It is understood Falloon's version of events is that he was at a party several weeks ago and briefly left his phone unattended - and acquaintances then used it to send the sexual image in question.
The Herald understands the image was not of himself but was pornographic in nature.
Police confirmed last night that an investigation began after receiving a report of an individual sending an "unsolicited image".
On Tuesday morning, as further allegations emerged, National Party leader Judith Collins told media she no longer trusted her MP, with Falloon's story continuing to change and his explanation "clearly a lie", and said she expected him to immediately resign.
Minutes later, Collins told Newstalk ZB she'd received confirmation from Falloon that his resignation was effective immediately.
Had Falloon stayed on until the September election he could have received a further $60,000 from the taxpayer.
This entailed about two months' of his $163,000 base MP salary ($27,000), plus a further three months' pay after the election or $40,000 in total - a perk afforded all MPs who have resigned, or are voted out - minus 10 per cent due to Covid-19-related cuts.
But in announcing his immediate resignation, Falloon forfeited those entitlements.
A spokeswoman for Parliamentary Services said MPs were only entitled to the additional three months' salary if (among other requirements) they were a member "immediately prior to the dissolution of Parliament", which would be on August 12.
In 2017 questions were raised around disgraced Clutha-Southland National MP Todd Barclay, who received $80,000 in taxpayer money despite resigning three months before that year's election, after a revelation he told then-Prime Minister Bill English about recording a staff member while publicly saying he had not done so.
Falloon is the 13th National MP to announce their resignation, although the 12 others will do so at the September election so will continue to collect their salaries plus three months' pay after polling day.
They will all be reduced a minimum 10 per cent due to Covid-related pay cuts that came into effect July 9 and are to last six months.
The collective payouts amount to roughly $1.4m, based on base MP rates and not including extra payments from committee roles.
The payouts range from about $238,000 for Wairarapa MP Alastair Scott, who announced his resignation in June last year; to about $61,000 for departing former deputy leader Nikki Kaye, who announced her resignation last week on July 16.
NATS HEADING TO THE EXIT DOOR
• Andrew Falloon — Rangitata: First-term electorate MP for the electorate centred on Ashburton. Previously worked for Act leader Rodney Hide, ministerial adviser to Steven Joyce. Was National's spokesman for biosecurity, associate agriculture, associate economic development, and associate transport. Announced resignation effective immediately on Tuesday after sending sexual image to a young woman.
• Nikki Kaye — Auckland Central: Elected in 2008 in the traditional Labour seat. Minister of Education, ACC, Civil Defence, Food Safety, Youth, Youth Affairs. In September 2016, stepped down as a minister to fight breast cancer, then returned. Deputy leader to Todd Muller from May 2020. Lost it when he resigned on July 14, announced retirement July 16.
• Amy Adams — Selwyn: Elected in 2008, Minister for Social Investment, Social Housing, Justice, Internal Affairs, Housing, Environment, Courts, Communications and Information Technology, Communications and Broadcasting. Lost leadership bid to Simon Bridges in February 2018 but became finance spokeswoman. Announced intention to retire in June 2019 but rescinded decision in May 2020 when Muller offered her Covid-19 policy co-ordinator. Re-announced retirement on July 16.
• Jian Yang — List: First entered Parliament in 2011 and a list MP since. Admitted during 2017 election campaign to a former role teaching English at a Chinese spy college. Apart from one press conference, refused media requests. Announced retirement on July 10.
• Hamish Walker — Clutha-Southland: First elected in 2017 and was forced to announce his retirement on July 8 after admitting to passing on lists of Covid-19 patients acquired from former party president Michelle Boag to several news outlets.
• Paula Bennett — Upper Harbour: Elected in 2005 as a list MP, Deputy Prime Minister under Bill English. Minister of Climate Change Issues, Disability Issues, Local Government, Police, Social Development, Social Housing, State Services, Tourism, Women and Youth Affairs. Deputy leader under Simon Bridges from 2018 until the Todd Muller leadership coup in May 2020. Announced retirement on June 29, before Muller resigned.
• Anne Tolley — East Coast: Elected in 1999 as a list MP for one term. Returned in 2005 as East Coast MP, a seat she has held since. Minister for Children, Corrections, Education, Education Review Office, Local Government, Police, Social Development and Tertiary Education. Announced retirement on June 27.
• Nicky Wagner — List: Entered Parliament in 2005, claimed the Labour stronghold of Christchurch Central for two terms, 2011- 2017. Has been a minister outside Cabinet, holding Customs, Disability Issues, Greater Christchurch Regeneration and Statistics. Announced retirement on February 11.
• Sarah Dowie — Invercargill: First elected in 2014. Spokeswoman for conservation. Revealed to have had an affair with former National MP Jami-lee Ross. Announced on February 11 she would retire - for reasons unrelated to Ross.
• David Carter — List: First elected in Selwyn in 1994 for two terms, a list MP since 1999. Became Minister for Primary Industries, Agriculture, Local Government, Forestry and Biosecurity. Speaker from 2013- 2017. Announced retirement on February 2.
• Maggie Barry — North Shore: The former broadcaster and gardening show host was elected in 2011. Was the Minister for Arts, Culture and Heritage, Seniors and Conservation. Announced retirement November 5.
• Nathan Guy — Ōtaki: First elected in 2005, Minister of Veterans' Affairs, Racing, Primary Industries, National Library, Internal Affairs, Immigration, Civil Defence, and Archives. Announced retirement in July 2019.
• Alastair Scott — Wairarapa: First elected to Parliament in 2014. Announced plans to retire in June 2019.