Some MPs in the junior Coalition partner are already discussing who could replace Mr Joyce as Nationals leader if he stepped aside; NSW MP Michael McCormack, Victorian Darren Chester and Queensland senator Matt Canavan are the names most commonly bandied about.

However, there is little appetite for a change of leader within the Nationals and none of those three MPs is actively positioning.

But Nationals MPs recognise just how seriously Mr Joyce has been damaged by the revelations that he had an affair with a former staffer in his office, Vikki Campion, and that the pair are expecting a baby.

Fairfax Media spoke to seven Nationals MPs on Sunday about Mr Joyce's political future, ahead of a scheduled party room meeting on Monday morning.

Allies and enemies of the New England MP praised the job he had done in representing country voters, but questioned whether he could ride out the damaging revelations.

One of those MPs, an ally of Mr Joyce, described him as a "once-in-a-generation leader" but added that "he won't survive this if some evidence emerges of travel allowance misuse".

Another ally said that "everything will be all right unless he has done something wrong with his travel".

A third Nationals MP predicted "it only gets uglier from here. We will get through it, I'm not sure that he will."

Mr Joyce told the ABC's 7.30 program last week that he had not used taxpayer-funded trips to conduct his relationship with Ms Campion.

"We have made sure that private matters remain private and anything on a public account has been to do with my work as a politician," he said.

Labor has vowed to pursue Ms Campion's subsequent appointments - she moved from the Deputy Prime Minister's office to that of his cabinet colleague Senator Matt Canavan and then on to Nationals MP Damian Drum's office - as well as any potential cases of misuse of taxpayers' money.

But Queensland MP Michelle Landry - the only Nationals MP willing to speak on the record on Sunday - defended Mr Joyce. She said she felt for Natalie Joyce and the couple's four daughters, and praised the work Ms Campion had done.

"I believe Barnaby can survive this, we are a united team, we are a big family," she said.

"My office has worked quite closely with Vikki. I've found her very dedicated to her job, she was very good at it. She did a lot for the Nationals, getting us more hands-on with the Facebook and the Twitter."

"And when Matt lost his job [when Senator Canavan resigned from cabinet temporarily], every one of his staff members went to another office, it wasn't just her."

Government sources rejected outright reports that Prime Minister Malcolm Turnbull had personally approved Ms Campion's moves to different offices.

Finance Minister Mathias Cormann insisted on Sunday that Ms Campion's job placements were "absolutely above board".

"She's clearly somebody who is qualified to do the job and she was hired in certain positions based on merit and there's nothing really further to add," he told Sky News. "All of my advice is that everything was absolutely above board."

Deputy Labor leader Tanya Plibersek said the opposition would keep asking questions about how taxpayer funds had been used.

Environment Minister Josh Frydenberg was asked on Sunday whether there should be an audit of Mr Joyce's travel expenses, after the Deputy Prime Minister repaid part of a rental car bill for a journey that reportedly involved Ms Campion.

Mr Frydenberg said that Mr Cormann had said "all the appointments were made on merit and the appropriate processes were followed".

Mr Joyce left a family holiday on the Sunshine Coast in January last year to return to Canberra to deal with an outbreak of white spot disease, which was hitting Australia's prawn farms.

Instead of flying directly back to Maroochydore to rejoin his family, Mr Joyce hired a car and drove back over several days - costing taxpayers $1648. He later repaid $708 because he was only entitled to claim the cost of returning to his home base of Tamworth, not to the Queensland coast.

The ABC has reported the journey involved Ms Campion although Mr Joyce's office on Sunday refused to answer further questions about the trip.

A spokesman for Mr Joyce said that under parliamentary rules, Mr Joyce was not entitled to fly to Maroochydore where he had been holidaying when the biosecurity outbreak occurred.

He was only entitled to fly to his home base of Tamworth at roughly the same cost – if not more – to the hire car component. That would still have placed him 650 kilometres short of where he needed to go.

Do you know more? Email: jmassola@fairfaxmedia.com.au