The outgoing MP released a lengthy statement on Wednesday afternoon saying he made the decision with a heavy heart and describing himself as "an imperfect human".

"A serious allegation has been made against me via the process of parliamentary privilege and the immediate leaking of my name to the media associating it with the speech - an action with clear political intent by that member of parliament and her support team to politically and personally damage me outside of the due process already underway," he said.

"I vehemently deny this allegation, which is devastating, unfair and unfounded.

"I will fight the allegation and expect to have my name cleared, as it just didn't happen. However the needs of the electorate come first and that is the approach I have always taken, often at great personal sacrifice such as the role of a member of parliament demands and many may not fully realise."

Mr Johnsen said the matter had taken a toll on his mental wellbeing, paying tribute to his staff and former Nationals colleagues.

"My partner and my family are my priority now, and I also thank them for their incredible love and support without which, at this stressful time, I could not do without," he said.

"I will make no further statements and ask that our privacy is respected now and in the future."

Nationals Leader John Barilaro and Gladys Berejiklian welcomed the news of Mr Johnsen's resignation a day after they both called for him to quit.

The resignation has plunged the Coalition into minority government, although the Premier is unlikely to lose her working majority in Parliament according to independent Lake Macquarie MP Greg Piper.

Mr Johnsen held the Upper Hunter electorate by just 2.2 per cent - making it vulnerable to the Shooters Fishers and Farmers - or even Labor which has never held the seat.

The pressure on Mr Johnsen to quit escalated on Tuesday after the ABC reported he'd allegedly offered a prostitute $1000 to have sex with him at his parliament house office, exchanged lewd texts with her and sent her an obscene video while sitting in the parliamentary chamber.

Last week Mr Johnsen revealed he was being investigated by police after the same sex worker accused him of raping her at the Blue Mountains in 2019. After that Mr Barilaro stripped him of his role as parliamentary secretary, removed him from the Nationals and coalition partyrooms and suspended him from the National Party.

On Tuesday Mr Barilaro and a chorus of senior colleagues expressed their disgust at his alleged behaviour and urged him to quit parliament. Ms Berejiklian said she was "very relieved" to hear about his early exit despite the government being left in a precarious position.

"That's a good outcome and we look forward to moving forward," she said.

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Labor Leader Jodi McKay says the ALP will field a candidate in the by-election, insisting she's not deterred by the National's historic monopoly of the seat. She grew up in the Hunter region and said Labor would have "a red hot go" at winning the seat, which has been held by the Nationals for more than 90 years. "People know that I have this region firmly in my heart," she said.

"The Upper Hunter deserves a member of parliament who is there for them, who is supportive of them and is working for them."

Mr Piper told the Newcastle Herald that he had no plans to stop voting with the Berejiklian government, despite being "appalled and shocked" by the the rape and sexting allegations engulfing Mr Johnsen. "My support has always been for Berejiklian, I guess," he said.

"While I think there's some good members in the opposition, the question is are they in a good position to take the reins, lead and provide good, stable government.

"Well, I'd be reasonably sceptical about that."

Mr Johnsen was approached for comment.

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