One Nation senator Malcolm Roberts said he renounced his British citizenship before he nominated for Parliament, but Britain only confirmed it in December — more than five months after the poll.

Key points: Malcolm Roberts insists he has only ever considered himself to be an Australian citizen

Malcolm Roberts insists he has only ever considered himself to be an Australian citizen Senator Roberts said he sent three emails to British authorities saying he did not believe he was British citizen

Senator Roberts said he sent three emails to British authorities saying he did not believe he was British citizen Senator Roberts "is choosing to believe that he was never British", his spokesman said

Senator Roberts told Sky News he received a form from British authorities on December 5 last year saying he was not a citizen.

He said he was "very confident" he could prove he was not a dual citizen. But he has not released the paperwork.

"When I heard of Larissa Waters' news last week, it was early last week, I was in Perth with the banking inquiry that I'm a chair of and I didn't have my documents with me," Senator Roberts said.

He insisted he has only ever considered himself to be an Australian citizen, but took steps before nominating in case he had dual citizenship because his father was Welsh.

Senator Roberts said he sent three emails to British authorities on June 6 last year saying he did not believe he was a British citizen, but if he was he renounced it.

That was before nominations closed.

But he said it took months of badgering to get Britain to confirm his status and that did not happen till well after the election.

He argued he complied with the rules by taking all reasonable steps before he nominated.

Earlier on Thursday, his spokesman Sean Black was quoted in Fairfax media saying Senator Roberts "is choosing to believe that he was never British".

"He is preferring to believe that he was never British because he has no allegiance or exercised any citizenship arrangement," he was quoted as saying.

"There is nothing wrong or incongruent with Malcolm Roberts putting his hand up and saying, 'as far as I'm concerned I'm not British, never was' — the British Government may have a different view."

The ABC has asked Mr Black to explain what it means to say "he is choosing to believe that he was never British".

Meanwhile, Pauline Hanson said she had to laugh at the media's "witch-hunt" of Senator Roberts.

"I can hand-on-heart assure everyone that Malcolm is not a dual citizen," she wrote on Facebook.

"I saw firsthand his renouncement of UK citizenship before he became a candidate for the Senate."

Greens Scott Ludlam and Larissa Waters have quit the Senate after discovering their dual citizenship recently.