Donald Trump says he has declassified documents about the origin of the Mueller probe, which may include a conversation former foreign minister Alexander Downer had with an official from the US President's election campaign.

Key points: Mr Trump declassified documents that prompted Robert Mueller's investigation into Russian collusion

Mr Trump declassified documents that prompted Robert Mueller's investigation into Russian collusion It's believed these documents could relate to a meeting Mr Downer had with a Trump campaign official in 2016

It's believed these documents could relate to a meeting Mr Downer had with a Trump campaign official in 2016 The US Attorney-General has the power to reveal the documents publicly, drawing criticism he will present misleading information favouring Mr Trump

The US leader addressed reporters on the White House lawn, telling the press he authorised his Attorney-General William Barr to examine what could be "millions of pages".

"I hope he looks at Australia and I hope he looks at Ukraine," Mr Trump said.

"I hope he looks at everything, because there was a hoax that was perpetrated on our country.

"Somebody has to get to the bottom of it."

Mr Trump alleges Robert Mueller's probe into Russian collusion was payback by "sore losers" upset by his election win.

An Aussie diplomat and a Trump campaigner

It is possible the documents could include those surrounding a meeting Mr Downer had with Trump campaign adviser George Papadopoulos at a London wine bar in 2016.

Mr Downer, then Australia's High Commissioner to the UK, said he asked Mr Papadopoulos whether he believed Mr Trump would defeat Hillary Clinton in the presidential election.

"He said he was confident he could," Mr Downer said.

"He said one of the reasons was that the Russians might release some information which could be damaging to Hillary Clinton."

Mr Papadopoulos' comment was concerning enough that Mr Downer and political counsellor Erika Thompson decided to send a cable back to Canberra about the exchange.

The cable outlined that Mr Papadopoulos claimed to be aware of a Russian attempt to discredit Mrs Clinton, something he denies.

Eventually, the cable was passed on to US intelligence, included in an investigation of Mr Trump's campaign and its ties to Russia.

Investigating the investigators

On Thursday, Mr Trump gave Mr Barr full authority to publicly disclose information about the origins of the investigation he repeatedly dismissed as a "hoax".

He said it was a bid to "investigate the investigators" who were behind an "attempted coup" in the White House.

Mr Trump said he wanted Mr Barr to "get to the bottom" of Russian collusion probe. ( AP: Andrew Harnik )

US Director of National Intelligence Dan Coats said the country's spy agencies would provide the Justice Department all appropriate information for its review of intelligence activities related to Russian interference in the 2016 election.

"Much like we have with other investigations and reviews, the intelligence community will provide the Department of Justice all of the appropriate information for its review of intelligence activities related to Russian interference in the 2016 presidential election," Mr Coats said in a statement on Friday.

"As part of that process, I am confident that the Attorney-General will work with the intelligence community in accordance with the long-established standards to protect highly-sensitive classified information that, if publicly released, would put our national security at risk.

"The intelligence community will continue to faithfully execute its mission of providing timely, apolitical intelligence to the President and policymakers."

Fears of manipulation

But Mr Trump's critics are wary of leaving the decision of what intelligence to release — and what should remain hidden — in Mr Barr's hands.

Mr Barr is a staunch Trump defender who Democrats say spun the Mueller report in Mr Trump's favour, playing down aspects suggesting possible criminal conduct.

Mr Mueller has also complained to Mr Barr about his handling of the release of the report.

David Kris, former head of the Justice Department's national security division, said it was "very unusual — unprecedented in my experience — for a non-intelligence officer to be given absolute declassification authority over the intelligence".

Chairman of the House intelligence committee, Adam Schiff, vowed to conduct oversight of any effort to selectively reveal and distort classified information or manipulate the declassification system.

"The clear intent of this abuse of power is to override longstanding rules governing classified information to serve the President's political interests, advance his 'deep state' narrative, and target his political rivals," he said.

But one of Mr Trump's congressional allies, Mark Meadows, said the directive reflected the President's pledge for an open and transparent investigation.

"Americans are going to learn the truth about what occurred at their Justice Department," he said on Twitter.

Concerns have been raised about the powers afforded to Mr Barr. ( AP: Andrew Harnik )

ABC/AP