An ABC reporter has alleged that the New South Wales opposition leader, Luke Foley, put his hands inside her underpants at a Christmas function in 2016.

Foley denied the allegations in a brief media conference at around 5.30pm on Thursday and said he expected to take legal action in response.

The allegations had quickly prompted an internal push to remove Foley. One backbencher had called on him to resign and said she would call for a spill if he did not. Senior Labor sources also said his position was “unsustainable” just five months out from the election.

Ashleigh Raper, who previously worked as a reporter inside the state parliament, released a statement on Thursday addressing reports that Foley had acted inappropriately at a function in November that year.

Foley had previously denied allegations that he harassed a journalist at the Christmas party, after they were aired last month by a Liberal MP under parliamentary privilege. He reiterated that denial on Thursday.

The ABC statement said the reporter had not wanted to make a complaint and had only now come forward because of “media and public pressure”. The ABC said it would not be commenting further.

Luke Foley threatens to dish dirt on ministers in mounting harassment row Read more

Late in the evening after the function, she said, the opposition leader had approached her and a group of people to say goodnight.

“He stood next to me,” she said. “He put his hand through a gap in the back of my dress and inside my underpants. He rested his hand on my buttocks.

“I completely froze.”

Raper said the encounter had been witnessed by the journalist Sean Nicholls, who was then the state political editor at the Sydney Morning Herald and is now an ABC journalist.

“Mr Foley then left the bar,” she said. “Sean and I discussed what happened. As shaken as I was, I decided not to take any action and asked Sean to keep the events in the strictest confidence. He has honoured that.”

Raper said in the statement that she had been contacted by Foley last Sunday. In a 19-minute phone conversation he had said: “I’m not a philanderer, I’m not a groper, I’m just a drunk idiot.”

She said Foley had apologised and said he was “full of remorse for his behaviour”.

“He told me that he had wanted to talk to me about that night on many occasions over the past two years because, while he was drunk and couldn’t remember all the details of the night, he knew he did something to offend me,” she said.

Raper said Foley had told her he would resign as the leader of the NSW Labor party on either the next day or Wednesday 7 November.

“He said he couldn’t resign on the Tuesday because it was Melbourne Cup day and he didn’t want to be accused of burying the story,” she said.

On Tuesday, however, she said he had contacted her again and changed his position, saying he had received “legal advice” not to resign.

“He repeated his apology and told me he owed me ‘a lot of contrition’,” she said.

“He informed me he’d received legal advice not to resign as opposition leader. He indicated he intended to follow that advice.”

The release of the statement prompted calls from within the Labor party for Foley to resign. Trish Doyle, the Blue Mountains MP, released a statement on Thursday afternoon threatening to call a spill of the party leadership if Foley did not resign.

“Politics, like the entertainment industry, is lagging far behind the rest of society in its handling of workplace sexual harassment and bullying,” she said.

“We need a significant cultural shift in society so that women can feel safe and so that when they make a complaint it is taken seriously.

“I am concerned that this issue has drawn out and caused such distress and anguish for the journalist at the centre of it.

“In my view, Mr Foley’s position is untenable and he must resign today … In the event that he refuses to resign, I will call for a spill of the NSW Labor leadership to resolve this issue.”

In her statement Raper said she had chosen not to complain for “a number of reasons”.

“It is clear to me that a woman who is the subject of such behaviour is often the person who suffers once a complaint is made,” she said.

“I cherished my position as a state political reporter and feared that would be lost. I also feared the negative impact the publicity could have on me personally and on my young family.

“This impact is now being felt profoundly.”

She said she had been contacted by another reporter about the matter earlier in the year and subsequently informed ABC news management about the encounter.

“I told them I didn’t wish to make a complaint or for any further action to be taken,” she said. “They respected my request for privacy and have offered me nothing but their absolute care and support.”

Situations like mine should not be discussed in parliament for the sake of political point scoring Asheigh Raper

The encounter came to broad public attention when the Liberal MLC David Elliott raised the matter it in NSW parliament last month, something Raper said happened “occurred without my involvement or consent”.

Elliott did not respond to requests for comment.

At the time, Foley labelled the accusations “a smear”.

Raper said: “There are three things I want to come from my decision to make this statement.

“First, women should be able to go about their professional lives and socialise without being subject to this sort of behaviour. And I want it to stop.

“Second, situations like mine should not be discussed in parliament for the sake of political point scoring. And I want it to stop.

“Third, I want to get on with my life.”

Guardian Australia understands that Michael Daley, the current deputy, is a leading contender to replace him.

“We’ll be supporting Michael Daley for a number of reasons, he’s the deputy, we want a smooth transition in what has taken all of us off guard,” a senior Labor source said.

“We’re going into an election, we want stability. He can provide that.”

Asked before the resignation whether there was any chance Foley could remain as leader, the Labor source responded: “No. It’s a pretty damning statement … You don’t get much clearer. It’s pretty unequivocal. Where do you go from there?”

One shadow minister said Foley’s position was clearly “unsustainable” and that “he will have lost all support.”

She said Daley would be the “top choice” to replace Foley if he resigned. “He’s solid as a rock,” she said.

The next NSW state election is due to be held in March 2019.

The two major parties were neck and neck last month, according to a ReachTel poll for the Sun-Herald. Their primary vote was locked at 50:50, and Foley had beaten Gladys Berejiklian as preferred NSW premier, polling at 50.2%.

A third Labor source said changing leaders might not be enormously damaging because Foley was relatively unknown.

“Moving from a leader in that context isn’t quite as catastrophic,” he said.

He told Guardian Australia just after 3.30pm that he was expecting Foley to make an announcement this afternoon but said the leader’s whereabouts was unknown. Even Foley’s own staff appeared to be unaware of his location, he said.

“No one thinks he should hang around,” he said. “We are all hoping there’ll be a statement this afternoon.

“He’s not in his office. They’re looking for him.”

The federal Labor leader, Bill Shorten, was asked about the allegations during a press conference on Thursday. He said he had not read Raper’s statement and that questions were best directed to Foley.

Prime minister Scott Morrison said the statement was “very shocking”.

“I’m not here to act as a judge and jury on these sorts of things but I must say I found those allegations very shocking and disturbing.”