Sexual harassment allegations against Nationals leader Barnaby Joyce have been described as “very serious” by Acting Prime Minister Mathias Cormann.

The Nationals confirmed late on Thursday its executive had received a formal sexual harassment complaint against Mr Joyce.

Federal party director Ben Hindmarsh said the complaint would be taken seriously, treated with strict confidentiality and given due process.

Mr Joyce said the allegation was “spurious and defamatory”.

“They should have been referred to police if they had substance,” he told the Nine Network on Friday.

Senator Cormann, who is filling in with Malcolm Turnbull in the US and Mr Joyce on leave, said any allegation of sexual harassment is very serious.

“Barnaby Joyce has denied the allegation, but it’s being investigated and subject to the outcomes of that investigation, obviously, the appropriate steps would follow,” Senator Cormann told reporters in Sydney.

The complaint comes as Mr Joyce faces a call from one of his colleagues to resign.

Victorian MP Andrew Broad became the first Nationals federal MP to call for the leader to quit, vowing to bring the matter to a head at a party room meeting in Canberra on Monday.

“At this point in time he should take a step back and stand down and be on the back bench,” Mr Broad told ABC radio.

Assistant Families Minister David Gillespie could put his hand up to replace Mr Joyce amid reports the NSW Nationals MP has indicated to colleagues his willingness to run if the party leader resigns.

Fellow NSW MP and Veterans Affairs Minister Michael McCormack, who recently gave an interview in which he dodged multiple questions about his support for Mr Joyce, is also considered a possible contender.

Speaking in Washington DC, Mr Turnbull declined to publicly back Mr Joyce, saying “leadership of the National party is a matter for the National party”.

His colleague Christopher Pyne also wouldn’t comment on the Nationals leadership, but told the Nine Network: “I support him as a cabinet colleague, of course I do, and as a friend.”

Despite mounting pressure, Mr Joyce is digging in after it was revealed earlier this month he had an affair with a former staffer who’s now pregnant with his child.

He told Fairfax Media he was confident he would still be leader after Monday’s party meeting.

“Andrew Broad has expressed these sentiments before,” Mr Joyce said.

“The proper venue for the discussion of these issues is within the party room, which we will do on Monday.”

Mr Broad or another MP could move a motion calling for a leadership spill or a motion calling on Mr Joyce to step down.

Labor frontbencher Anthony Albanese again called for Mr Joyce to resign, criticising his decision to do media interviews during a week of leave.

“The deputy prime minister’s first job is to deputise for the prime minister. He himself has admitted this week he can’t do that job,” he said.
”He just should go.”

-AAP