A National MP has resigned after allegedly sending unsolicited sexual images to young women, plunging the opposition National party into crisis once more, less than two months before the election.

Andrew Falloon, a first-term South Island MP, has left parliament after announcing he would not re-contest his seat in Rangitata in September’s election amid allegations that he sent an explicit image to a 19-year-old university student.

New Zealand police said they were reopening inquiries into the allegations, after earlier saying they had found an initial complaint did not meet the threshold for prosecution.

Police said in a statement they “have today been made aware of further allegations” raised by Judith Collins, the National party leader, and were “now seeking more information in relation to these matters”. They urged anyone with information to come forward.

On Tuesday morning, Falloon took heed of a call from Collins to resign from parliament immediately.

“I am absolutely appalled by what I have been hearing this morning and finding out from members of the public,” she said, labelling him a disgrace.

News outlets Stuff and Newshub have reported further allegations that Falloon, 37, has acted similarly to other young women, and Collins expressed concern that it could be part of a “pattern of behaviour”.

The National party was alerted to allegations about Falloon’s behaviour by Jacinda Ardern’s office, after a complaint sent to the PM’s office was privately re-directed to the opposition.

Falloon hasn’t spoken since news of his behaviour came to light, issuing a statement on Monday afternoon that a friend’s recent suicide had resurfaced personal traumas.

“As I noted in my maiden speech three years ago, when I was younger I lost three close friends to suicide,” he said in the 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 he had been escorted from parliament to the airport by an MP and picked up from the airport by another MP, saying he was deeply mentally unwell.

“I was very deeply concerned for the welfare of the young woman involved, but I was also deeply concerned with the safety of Andrew Falloon. He is still a human being,” she said.

The scandal is another sorry chapter for the opposition less than two months before a national election.

Last week, Todd Muller shocked his party room by resigning the leadership after just 53 days in the job – the shortest spell for any elected party leader in New Zealand history.

Australian Associated Press contributed to this report