Australia's relationship with the US will not be damaged by the Trump presidency because "no one president has enough time to screw it up", former FBI director James Comey has told 7.30.

Despite describing Mr Trump as "morally unfit for the role", Mr Comey said he believes the US will remain a reliable and trustworthy partner for Australia during Mr Trump's time in office.

"I know the extent and culture of the relationship between the two countries," Mr Comey said.

"It'd be hard to screw up the relationship between the United States and Australia.

"No one president has enough time to screw it up, because it's so longstanding and so beneficial to both sides."

In an interview with 7.30's Leigh Sales in Manhattan, Mr Comey said he felt "sick" at the thought that his handling of the Hillary Clinton email affair may have affected the 2016 US election.

When asked what role he personally played in the election of Donald Trump, Mr Comey said: "I hope and pray none."

"The notion that we had an impact on the election makes me sick to my stomach," he said.

James Comey says he felt "sick" at the thought he helped Donald Trump win the 2016 US election. ( Reuters: Joshua Roberts/AP: Evan Vucci )

During the 2016 campaign, Mr Comey twice commented publicly on the FBI's investigation into Hillary Clinton's use of a private email server while secretary of state.

He reopened the case in the crucial final days of the campaign.

But he has defended his decision not to speak about the investigation into Russian interference, which may have involved Mr Trump's campaign staff, because it was early days and "we didn't know what we had".

"It just makes it more painful. You've still got to do the right thing and choose as — again, as I thought about it, between bad and catastrophic. You've got to choose bad everyday over catastrophic.

"The notion that because of the choices we had to make we had an impact, leaves me feeling ill. Now, it may sound odd to say, but that wouldn't change the way I thought about the decisions."

Firing Mueller 'wouldn't stop investigation'

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Mr Trump has spent the past week using Twitter to rail against Mr Comey, who has been promoting his new book A Higher Loyalty: Truth, Lies and Leadership, which at times reflects poorly on Mr Trump.

The US President has hit back, branding him a "proven leaker & liar", "untruthful slime ball", "slippery", "terrible", "not smart!" and "the WORST FBI Director in history, by far!"

The firing of Mr Comey in 2017 prompted the establishment of a special prosecutor — another former FBI director, Robert Mueller — to investigate the allegations of Russian interference.

But when asked whether the US President could also fire Mr Mueller, Mr Comey said it was possible, but "the practical effect would be almost nil".

"Because somebody will pick it up, and if that person gets fired the next person will pick it up, and so until the buildings are vacant, it wouldn't stop.

"So I think it would be bad for a lot of reasons to fire the special prosecutor here, it would be an attack on the rule of law.

"But it would actually be dumb because it wouldn't be stopping the goal of stopping the investigation."

Mr Comey's book, A Higher Loyalty, triggered a backlash from Mr Trump, who called him a "slimeball". ( Reuters: Mike Segar )

Until recently, Mr Comey was a longtime registered Republican, but today he told 7.30 he did not vote in the 2016 election.

As for his place in history, the New Jersey-born career investigator said he does not "really care much".

Watch the full interview with James Comey tonight on 7.30.