Two federal Nationals MPs have now gone public with doubts over the viability of Barnaby Joyce's leadership ahead of a much-anticipated party room meeting on Monday.
Expectations are growing that Joyce may quit the Nationals leadership on Friday.
Mr Joyce is due to hold a media conference in Armidale at 2pm AEDT, but his office would not confirm speculation of his resignation.
Nationals MP Andrew Gee suggested his support would depend on the results of an investigation into a recent sexual harassment allegation against Joyce, which acting prime minister Mathias Cormann described as "very serious".
File: Barnaby Joyce, Nationals Member for Calare Andrew Gee and Minister for Agriculture and Water Resources David Littleproud Source: AAP
"Until I have clarification regarding some of the issues about Barnaby, I won't be able to back him but I'm also not in a position to back any challenger," Mr Gee said in a statement to Channel Nine.
"As things stand, all bets are off, I'm backing the electorate and not anyone else, and we'll just have to see what next week brings".
Mr Joyce denies any wrongdoing and has called the allegations "defamatory". The embattled Nationals leader is reportedly planning to speak to the media again on Friday afternoon.
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.
"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 was 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.
Mr Gee's remarks come after a stronger condemnation from another Nationals MP, who openly called on Mr Joyce to quit. Victorian MP Andrew Broad vowed to bring the matter to a head at the scheduled 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."
- with AAP
