Australia's ambassador to the United States Joe Hockey has disputed a claim by a Republican senator that Alexander Downer played an orchestrated role in starting the Russia inquiry.

Key points: Joe Hockey rejected Lindsey Graham's claims that someone "directed" Alexander Downer to set up a meeting with George Papadopoulos.

Joe Hockey rejected Lindsey Graham's claims that someone "directed" Alexander Downer to set up a meeting with George Papadopoulos. Earlier this week, Scott Morrison said Mr Downer's actions were not under investigation

Earlier this week, Scott Morrison said Mr Downer's actions were not under investigation Downer has rejected any suggestion he was part of an anti-Trump conspiracy

Senator Lindsey Graham has controversially suggested that Mr Downer, the former high commissioner to the United Kingdom, was "directed" to seek a meeting with Trump campaign adviser George Papadopoulos.

He wrote that "US intelligence communities" accepted "information from an Australian diplomat who was also directed to contact [Mr] Papadopoulos and relay information … to the Federal Bureau of Investigation".

That meeting, in a London wine bar, helped spark the Mueller inquiry into Russian election interference, which was repeatedly denounced as a "witch hunt" by the US President and his supporters.

Donald Trump's supporters often cite a conspiracy theory that the intelligence services of several countries, including Australia, had a secret plan to derail Mr Trump's election, and the origins of the Mueller inquiry are now the subject of a Department of Justice investigation under Mr Trump's Attorney-General William Barr.

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In a letter to Senator Graham, published on Twitter, Mr Hockey pledged Australia's continued support for the Justice Department investigation but rejected the senator's characterisation of Mr Downer's role.

"In your letter you made mention of the role of an Australian diplomat. We reject your characterisation of his role," Mr Hockey wrote.

Earlier this week, Prime Minister Scott Morrison confirmed Australia was ready to help Mr Barr but said: "There have been no requests, or no recommendations that have been made to me, that needed my intervention".

Mr Morrison said Australia would not release documents that could affect the national interest, but would not elaborate on what that might entail.

He said it would be uncommon for the country to provide diplomatic cables, and said he did not know if US officials had requested any.

"Not to my knowledge, no. But these are conversations that are happening between officials. These go to events well before I was Prime Minister."

Mr Morrison said Mr Downer had acted appropriately in his deals as high commissioner.

"We've got nothing to hide, we're not the subject of this investigation nor are we a party to it."

After the May 2016 meeting, Mr Downer said Mr Papadopoulos was confident that Mr Trump would win the upcoming presidential election, because the Russians might use damaging material they had on his rival Hillary Clinton.

Mr Downer sent this information in a cable back to Canberra, and Australia later passed it on to US intelligence agencies when it became known that the FBI suspected that the Russians had hacked Mrs Clinton's emails.

Mr Downer has rejected claims he was part of a conspiracy to discredit Mr Trump.

"This sort of idea that there is a kind of a ASIS-ASIO-MI6-MI5-FBI-CIA-Ukrainian Government conspiracy to bring down the Trump administration, that this is treason, that I should be in Guantanamo Bay … I mean it's a little bit sad that people take that kind of thought seriously," he told the ABC's Russia If You're Listening podcast.

George Papadopoulos has accused Alexander Downer of being a spy. ( Reuters: Yuri Gripas )

Mr Papadopoulos this week accused Mr Downer of being a "wannabe spy" and a "Clinton errand boy", who would soon be "exposed on the world stage", and suggested Australia was trying to "obstruct [Mr Barr's] investigation".

Mr Papadopoulos served a 14-day prison sentence last year after admitting he had lied in a 2017 interview with the FBI, hindering their investigation.

Democratic sources have told the ABC that Australia's involvement in the Barr inquiry will not affect the country's relationship with the Democratic Party so long as Australia plays the Justice Department's requests with a straight bat.

The White House, Senator Graham and Alexander Downer did not respond to requests for an interview. Mr Hockey declined to comment.