Australia’s ambassador to Washington, Joe Hockey, offered to participate in a controversial probe established by Donald Trump aimed at discrediting the Mueller inquiry before the Americans asked.

The Morrison government has also nominated the secretary of the Department of Foreign Affairs and Trade, Frances Adamson, as the primary point of contact in Canberra officialdom for the Barr inquiry, which is viewed in the United States as a highly charged partisan exercise. Adamson met William Barr, Trump’s attorney general, on 21 September, during the prime minister’s recent visit to Washington.

The timelines, which shed new light on Australia’s participation in the Barr probe, emerged during a Senate estimates hearing in Canberra on Thursday.

On 25 May Trump blasted Australia before leaving on a trip to Japan, declaring he wanted Australia’s role in setting off the FBI inquiry into links between Russia and his election campaign examined by Barr.

The Mueller investigation began after Australia’s former foreign affairs minister Alexander Downer was allegedly told by George Papadopoulos, then a Trump campaign aide, that Russia had obtained damaging information about Hillary Clinton from her emails. Downer then informed US authorities.

Trump said in late May he hoped Barr would “look at the UK and I hope he looks at Australia and I hope he looks at Ukraine. I hope he looks at everything, because there was a hoax that was perpetrated on our country.”

Australia’s foreign affairs minister, Marise Payne, then signalled publicly that the government was prepared to contribute to the Barr investigation if asked. Payne said at the time Australia had not yet been asked but “we would, of course, consider such a request were it to be made”.

Hockey went further, writing to Barr, copying in the White House chief of staff, Mick Mulvaney. The ambassador said: “The Australian government will use its best endeavours to support your efforts in this matter.

“While Australia’s former high commissioner to the United Kingdom, the Hon Alexander Downer, is no longer employed by the government, we stand ready to provide you with all the relevant information to support your inquiries.”

While officials from Morrison’s department declined to answer questions about the Barr process in a separate hearing on Monday, Dfat officials said on Thursday it had been Hockey’s idea to signal a more forward-leading position to the Trump administration.

Payne told Thursday’s hearing Hockey had first flagged that idea with her. Asked by Labor’s Senate leader, Penny Wong, whether the evidence was Hockey had agreed on Australia’s behalf to participate in the Barr probe without being asked, the secretary of the department Frances Adamson said: “That’s correct.”

Hockey then met privately with Barr, and Adamson met with the US attorney general in late September, having been nominated as the Canberra point of contact. There have been at least four meetings between Australian officials and Americans involved in the Trump investigation.

Dfat officials declined to answer questions about what had been discussed, or whether Australia had supplied the investigation with diplomatic cables. Payne told Thursday’s hearing she had not discussed the Barr inquiry with Downer and neither had Morrison “to my knowledge”. Adamson said she had discussed the probe with Downer.

Officials declined to answer whether Downer had asked for either legal or financial support from the government in relation to the Barr probe on the basis the answer would be prejudicial both to a live investigation and to Australia’s relationship with the US. Wong objected to the lack of disclosure. “Why is that a secret?”

Officials also declined to answer whether Downer had relayed the insights from Papadopoulos to the American chargé d’affaires in London in July 2016. Guardian Australia revealed at the start of October that Downer had taken the then prime minister, Malcolm Turnbull, and senior colleagues by surprise when he shared information with his American counterpart in London.

Downer’s conversation with Papadopoulos was recorded in a diplomatic cable back to Canberra, according to senior sources.

But the first senior players in Canberra knew of Downer’s conversation with America’s top diplomat in London about Russia obtaining damaging information about Hillary Clinton from her emails was when the Australian government was contacted subsequently by the Federal Bureau of Investigation for more information. That diplomat-to-diplomat conversation was unauthorised.

Asked by Wong on Thursday whether Payne had confidence that Downer acted appropriately, the foreign minister said: “Yes senator.”

Hockey has previously flatly rejected a description by the Senate judiciary committee chairman, Lindsey Graham, of Downer’s “directed” role in relaying information to US authorities about the Clinton emails.

In a recent letter to Graham, the ambassador insists Downer wasn’t directed. “We reject your characterisation of his role. As you have requested, we will work closely with the attorney-general to resolve any misunderstandings in this matter.”