Nationals Leader John Barilaro and Premier Gladys Berejiklian welcomed the news, which has plunged the coalition into minority government.

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.

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 told media in the electorate on Wednesday.

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

"The feedback I get is there are many issues that have not been addressed in the Upper Hunter."

Mr Johnsen has been approached for comment.

Australian Associated Press