An adviser to One Nation senator Malcolm Roberts has threatened to report a journalist to police over his questions about the senator's citizenship.

A cloud remains over Senator Roberts' citizenship because a form showing he renounced British citizenship came five months after last year's election.

The constitution means he had to have renounced any other citizenship before he nominated.

Senator Roberts told Sky News last night he fulfilled that requirement because he contacted British authorities in June last year — just before he nominated — to tell them he renounced his citizenship.

But when a Fairfax journalist emailed questions arising from last night's interview, his staff member, Sean Black, responded: "Your persistent emails, despite requests to stop, is classified by our office as stalking."

"Any continued pestering or harassment from you or your colleagues of anyone, including me, from our office or organisation will be referred to the Queensland police," Mr Black's email said.

"This issue is now over. Stop the harassment. Please find another victim."

When the ABC requested the document showing he had renounced his British citizenship, Senator Roberts' office replied that the Senator had released it to "NewCorp [sic]" but not the ABC, and stated he would release it to a parliamentary review.

He did not say why he would not make it publicly available immediately.

It remained unclear whether Senator Roberts filled in the form available on the British Government's website to renounce his citizenship.

The six-page form and a guide to using it are both readily available online and there is an application fee of about $526.

Greens leader Richard Di Natale has called on One Nation to do the "honourable thing" and ask the Senate to refer the question of Senator Roberts' citizenship to the High Court.

Uncertainty fuels speculation

Senator Roberts told Sky News he received formal registration of his renunciation from the British Home Office in December.

The online information for applicants on the British Government website reads:

"You'll get a 'declaration of renunciation' if your application is successful. This will be your application form, officially signed and stamped." "The date your citizenship or status stops will be shown on the form."

Senator Roberts has not revealed when the form says his British citizenship stopped.

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He has said that he did not believe he had ever been a British citizen, despite online records showing that he had been.

The uncertainty makes it likely the High Court will be asked to consider his eligibility.

If Senator Roberts was eventually found to be ineligible, the most likely replacement would be One Nation candidate Fraser Anning.

But Mr Anning faces bankruptcy proceedings in the Federal Court — another cause for disqualification from office.

If Mr Anning was also ruled ineligible, the next candidate would be Judy Smith — Pauline Hanson's sister.