Deputy Prime Minister Barnaby Joyce is anticipating the birth of a child with his former media advisor Vikki Campion, according to reports.
Tabloid newspapers on Wednesday published front-page photographs of Mr Joyce's partner, aged 33, who appears heavily pregnant.
Mr Joyce, 50, already has four daughters with his ex-wife Natalie Joyce.
The senior Turnbull Government minister revealed his 24-year-long marriage had broken down last year during a parliamentary debate on same-sex marriage, which the Nationals leader strongly opposed.
"The current definition of marriage has stood the test of time – half of them fail, I acknowledge that,” he told the House of Representatives at the time.
"I’ll acknowledge that I'm currently separated, so that's on the record."
Mr Joyce, a conservative Catholic, defended "traditional" marriage throughout the same-sex marriage debate and eventually abstained from the vote in the Lower House.
His estranged wife Natalie Joyce told the Australian she was "deeply saddened by the news that my husband has been having an affair and is now having a child with a former staff member".
“This situation is devastating on many fronts," Ms Joyce said.
"For my girls, who are affected by the family breakdown, and for me as a wife of 24 years, who placed my own career on hold to support Barnaby through his political life."
Rumours of the relationship with the staffer have been circulating since the New England by-election in October, which Mr Joyce won by a strong margin after forced him to recontest the seat.
But today's story splashed across the News Corp publications will bring the matter into the public consciousness.
The couple have reportedly moved in together and their baby is apparently due in April, according to the Daily Telegraph.
Politicians reluctant to comment
A spokesman for Mr Joyce told SBS News the minister did not have a comment on the story.
Fellow Nationals MP George Christensen said he would not comment on his colleague's "personal issues".
"That’s not the sort of country we are in Australia, doing scandal-type exposes on politicians' personal lives," Mr Christensen said.
Dan Tehan, the minister for social services, said his "personal view" was that the relationship was a "private matter", speaking on ABC Radio.
The opposition has also declined to comment on the matter.
"The Labor party has made no public statement about Barnaby Joyce's personal situation for the last six months, despite the fact it's been well-known in this building, and we're not about to start today," senior Labor MP Chris Bowen told reporters at Parliament House on Wednesday.
The prime minister, Malcolm Turnbull, has told other media outlets he would not comment on the matter.
- with wires