Joyce, whose National Party rules alongside Prime Minister Malcolm Turnbull's Liberals, has been front-page news in Australia for two weeks since it emerged he had left his wife of 24 years for his younger former media adviser, who is now expecting their baby boy.
He was accused of "a shocking error of judgement" by Turnbull, with Joyce, who has four daughters, hitting back by calling the prime minister "inept".
Now the 50-year-old is facing a sexual harassment allegation, with his party confirming a complaint had been lodged, reportedly by a West Australian woman.
Joyce was cited by local media as describing the claim as "spurious and defamatory" and was expected to hold a press conference later Friday to address the issue.
He was due to be the acting prime minister this week with Turnbull meeting US President Donald Trump in Washington, but opted to take leave.
With Foreign Minister Julie Bishop also out of the country, the role has been assumed by Senate leader Mathias Cormann, who said any harassment claim must be taken seriously.
"Any allegation of sexual harassment is a very serious allegation," he told reporters Friday.
"I understand that a formal complaint has been made, and that that complaint is being investigated. I mean, at this point, that is really all that I have to say about it."
Colleagues of Joyce are reportedly growing increasingly frustrated with his handling of the love-child scandal.
He has opted to give several media interviews this week, at a time when he was expected to be on vacation and out of the spotlight.
A party meeting on Monday could decide his fate, with one of Joyce's party's backbenchers already publicly calling on him to quit.
A furious Turnbull, who relies on the smaller National Party to govern, has slapped a formal ban on sex between cabinet members and their staff in the wake of the Joyce affair.
He twice declined to offer support for his deputy when asked by reporters in Washington on Thursday, the Australian Broadcasting Corporation said.
The daily media headlines on the scandal have riveted the Australian public and sparked debate about workplace culture amid the global #MeToo movement against sexual harassment.
But it has also highlighted the perilous state of the coalition government, which just a few months ago survived a crisis over lawmakers' dual citizenship that threatened its wafer-thin parliamentary majority.
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