Papadopoulos's tactics mimic the use of conspiracy theories by the White House to stymie the Trump impeachment inquiry.

This month, Downer denied claims made by Trump loyalists that he had been "directed" to contact Papadopoulos in 2016.

"Of course I wasn't 'directed'," Downer told The Age and The Sydney Morning Herald.

Downer has also mocked Papadopoulos' conspiracy theories, such as when he revisited the London wine bar with his daughter, Georgina Downer and proclaimed he was spying on her for "CIA, FBI, M16, ASIS, ASIO and many more!!!"

Papadopoulos didn't immediately comment after filing the papers.

On Sunday he tweeted, "I love my state too much to see it run down by candidates like Hill. All talk, no action, and a bunch of sellouts."

He enters a field of at least three other Republicans and one Democrat.

The other Republicans are Navy veteran Mike Garcia, bank executive Angela Jacobs Underwood and Mark Cripe, who works for the Los Angeles County Sheriff's Department.

The seat was the last Los Angeles County to be held by Republicans before Hill's victory and was one of seven that Democrats flipped last year.

Hill announced her resignation on Sunday amid an ethics probe into allegations she had a relationship with a congressional staff member.

State Assemblywoman Christy Smith is the only Democrat in the race so far. She quickly criticised Papadopoulos on Tuesday.

"If he pled guilty to lying to the FBI – how do we know he'll tell us the truth?" Smith tweeted.

"We deserve someone from our community serving as our voice – not [Trump's] wannabe political hack!"