The Canberra Times on Wednesday revealed Simeon Boikov, a far-right "self-declared agent of Russia", was part of a delegation of anti-vaccine mandate protesters signed into Parliament by the United Australia Party leader on Tuesday.

Mr Boikov also once told the ABC's Four Corners that Russian opposition leader Alexei Navalny should have been "liquidated".

The Australian-born extremist - nicknamed "the Aussie Cossack" - leads the Zabaikal Cossack Society of Australia, a far-right nationalist group which supported Russia's invasion of Crimea in 2014.

Prime Minister Scott Morrison in 2018 personally intervened to protect Mr Kelly in a Liberal preselection battle for the seat of Hughes, before the MP defected to the crossbench in February 2021.

Labor home affairs spokeswoman Kristina Keneally described Mr Kelly as Mr Morrison's "captain's pick", and urged the Prime Minister to take responsibility for Coalition MPs who have also appeared at the rallies.

"It is deeply concerning that Craig Kelly judged it appropriate to sign an ultra-nationalist, with ties to the extreme right, into Australia's Parliament House," she said.

"Labor has been calling on Mr Morrison to take seriously security agencies' warnings that the far right and foreign interference are growing threats to Australia's national security."

Home Affairs Minister Karen Andrews declined to comment.

Parliament is shut to the public this week due to COVID-19, but MPs and staff are entitled to sign in guests for meetings.

A spokeswoman said the Department of Parliamentary Services does not comment on individual security matters, but ensuring visitors attended for "legitimate reasons" was the responsibility of the politician escorting them.

Mr Boikov, an Australian citizen, was entitled to enter Parliament and would have been able to do so freely but for COVID restrictions, Mr Kelly argued.

READ MORE: Far-right extremists 'cloaking themselves in' the anti-vax movement

The UAP leader did not respond when asked whether he would sign in other types of extremists on that basis.

"It would be a very dangerous situation and contrary to Australia's democratic principles if Australian citizens were denied access to Parliament House based upon some comments that they may or may not have made in the past," he said.

"Thankfully we have not arrived at that police state as yet."

The delegation was closely monitored by parliamentary security and the Australian Federal Police. They were largely contained to Mr Kelly's office, and were not left unaccompanied at any time.

Liberal senator James Paterson, chair of the powerful Parliamentary Joint Committee on Intelligence and Security, said who MPs chose to bring into the building "reflects on them".

"MPs' ability to sign guests into Parliament House is a responsibility which should be judiciously exercised with the safety and security of all building occupants in mind," he tweeted.

United States Studies Centre associate professor David Smith said the protests under way in Canberra were a combination of people who genuinely opposed mandates and feared the vaccine, and "cynical" actors taking advantage of them.

He said Mr Kelly, who was in a "marginal position" in Australian politics, and Mr Boikov were in a symbiotic relationship as they searched for attention.

"I would put someone like Simeon Boikov very much in the cynical category," he said.

"I don't think he's got a lot of attachment to democracy. He will latch on to anybody who will give him the publicity that he wants."

READ MORE: Explainer: Why are people protesting in Canberra?

Mr Boikov's presence in Parliament has sparked concerns that extremism is being normalised in Australia as it has been in the US, where anti-democratic conspiracies drove pro-Trump rioters in an attempt to overturn the 2020 election result.

But Mr Smith said Australia was protected by many institutional guardrails which do not exist in the US, preventing extremists from "taking hold".

Mr Smith said "apocalyptic" political rhetoric was not mainstream in Australia, where voters were also overwhelming pro-vaccine.

And the disciplined structure of its political parties, including MPs choosing their leaders, made the rise of an extremist outsider unlikely, he said.

"When someone like Craig Kelly gets too far out of line, his party will ultimately take action against him," he said.

"It wouldn't really matter how popular someone like him got, he could still be effectively kicked out of the Liberal Party.

"They probably should have done it sooner, but they ultimately did it."

Mr Kelly had a major international audience before he was banned from Facebook for spreading misinformation on COVID-19. He and Mr Boikov were more adept at using social media than mainstream political actors, Mr Smith said.

"They have found ways to mobilise sections of the of the public who usually feel excluded from politics, and they're not going away," he said.

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