Mr Downer gave an interview on Sky News on Sunday, where he criticised the style of US President Donald Trump, saying "I personally think that some of the language he uses is completely inappropriate for a head of government, particularly the most powerful government in the whole world".

Mr Downer also attacked the US media as biased towards the Democrats.

But his interview sparked a vitriolic response from a Twitter account attributed to George Papadopoulos, the former Trump adviser whose 2016 drinks meeting with Mr Downer in London set off the Mueller inquiry.

"Alexander Downer, the Clinton errand boy whose bizarre behavior at my meeting with him sparked the Durham probe, was on Australian tv today ranting like a lunatic against Trump and me. The probe is closing in on him. He's feeling the heat," the account @GeorgePapa19 tweeted on Monday.

Mr Downer appears at the Press Club on Tuesday to speak about Brexit but it is his role in the Mueller inquiry that has him in international demand.

At their 2016 meeting, Mr Papadopoulos told Mr Downer that Russia had dirt on Hillary Clinton. When Mrs Clinton's emails began appearing soon after, Mr Downer reported the meeting to Canberra, sparking the Mueller special council prove and plunging the Trump administration into a crisis.

Now, Mr Trump appears bent on revenge, and has set his Attorney-General, William Barr, to delve into how the Mueller probe began, in an investigation that stretches well beyond the US's borders to Australia, Britain, Italy and the Ukraine.

In Australia, Department of Foreign Affairs and Trade secretary Frances Adamson has been appointed as Australian contact for Mr Barr's investigation.

What hasn't been disclosed is whether Mr Downer himself will speak with Mr Barr - or has done so, and the extent to which Australian diplomatic documents, including the cables Mr Downer sent back to Canberra after meeting Mr Papadopoulos in London, will be shared with the US.

On Sunday, Mr Downer said he would not discuss the investigation publicly but the Australian government was cooperating fully with the US and he was cooperating fully with the Australian government.

"Whether I'm retired or not, I play for one team, and that team is Australia," he said.

Mr Downer's actions have sparked further questions since it was reported in the Guardian in October that not only did he report back to Canberra on his meeting with Mr Papadopoulos, he also spoke to the American charge d'affaires in London in July 2016, a conversation the Canberra reportedly didn't know about until contacted by the FBI.

Mr Downer has now become the subject of American right-wing conspiracy theories, fed by the @georgepapa19 twitter account, which has described Mr Downer as a "Clinton friend" who is "connected to the MI6, and private intelligence organizations in London".

Trump ally, Republican Senate leader Lindsey Graham upped the stakes when he wrote to Prime Minister Scott Morrison on October 2, in a letter that also went to the prime ministers of Italy and Britain, pointing to US law enforcement "(1) relying on a deeply-flawed dossier filled with hearsay and written by a biased, former United Kingdom intelligence officer; (2) receiving intelligence from an Italian "professor" who was directed to contact a low-level campaign adviser, George Papadopoulos to gather intelligence on the campaign; and (3) accepting information from an Australian diplomat who was also directed to contact Papadopoulos and relay information obtained from Papadopoulos regarding the campaign to the Federal Bureau of Investigation."

Mr Graham's letter appears to bounce off conspiracy theories of an international attempt to undermine Mr Trump, and prompted a strong response from outgoing Ambassador to the US, Joe Hockey, who tweeted his response to Mr Graham. "We reject your characterisation of [Mr Downer's] role," he said.

Mr Downer laughed at the conspiracy theories in May.

"The FBI and the State Department know that's not true ... As if somebody would ring me up and say, 'Will you go and spy on some clown called Papadopoulos who's a volunteer from the Trump campaign?'" he told the Sydney Morning Herald in May.

"Are you kidding ... If you wanted to spy on the Trump campaign, wouldn't you spy on a main player ... Steve Bannon or someone like that?"

Asked on Sunday about Mr Papadopoulos's Twitter attacks on him, he said, "The stuff I read on Twitter is unbelievable." People should find better ways to use the forum other than "10 different ways of abusing people you disagree with", he said.