Barnaby Joyce's leadership is under a cloud, as Nationals M-Ps warn one more revelation about his personal life could cost him the party's top job.
While there were calls for Mr Turnbull to urge Mr Joyce to step down, the decision rests with the National Party.
Columnist Miranda Devine reveals explosive new information in Rendezview about how Natalie Joyce found out about the deputy prime minister's affair with his former staffer Vikki Campion, and her reaction to it.
'Not the affair itself, that's nobody's business.
Mr Cassidy said "there might be something" but refused to comment saying it could not be substantiated.
Treasurer Scott Morrison told the ABC on Monday night: "There's no-one I know in the parliament who is a stronger advocate for rural and regional Australia".
'But this is not the end of it'.
O'Sullivan, who said he was not personally close to Joyce, has a reputation as a straight talker.
The Canberra insider, whose husband works for the Prime Minister, told the "sun is setting" on the Nationals leader's career.
Mr Joyce began an affair with his media adviser Vikki Campion - who is now pregnant with his child - and was advised previous year that it was inappropriate for her to continue working with him.
'I think his career is over, maybe not in the short term, but certainly in the medium term, ' she told the program.
"We're now starting to see in Australia rather serious threats and not just pretend threats - and that's just an incredible disappointment for all of us", he said.
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'That would be the finish if something came out about misuse, ' she said.
"It might be that there is no case to answer here, but I think the deputy prime minister and the prime minister should be prepared to answer questions about taxpayer money".
"He could have got out of it a bit better I thought", added Ms Savva.
Media reports have suggested Prime Minister Malcolm Turnbull was involved in approving the two appointments.
Mr Drum's taxpayer-funded staffing allocation was reportedly boosted to accommodate Ms Campion.
The Australian have reported that this property was provided for free for a local businessman (and known friend of Mr Joyce) Greg Maguire.More as this story develops.
Mr Joyce is now living with Ms Campion in Armidale, New South Wales.
Mr Joyce is facing questions over the process which saw his now-partner Vikki Campion take up a senior advisor job in the office of Nationals whip Damian Drum while Mr Joyce was embroiled in the citizenship fiasco a year ago.
"I hope that is the situation and let's see what pans out", Senator Williams said.
The ABC understands Mr Joyce's affair could prompt the Federal Government to consider tightening the ministerial standards to clarify the rules surrounding relationships.
Natalie Joyce said she felt "deceived" by the news that Ms Campion is pregnant with her husband's fifth child and claimed the relationship began when she was a paid staffer.