One Nation should do the "honourable thing" and ask the Senate to refer Malcolm Roberts' citizenship to the High Court, Greens leader Richard Di Natale says.

Key points: Greens leader says One Nation should ask High Court to rule on Senator Roberts

Greens leader says One Nation should ask High Court to rule on Senator Roberts Cloud remains over his eligibility because form shows he renounced British citizenship after election

Cloud remains over his eligibility because form shows he renounced British citizenship after election Senator Di Natale calls for independent audit into eligibility of all members of Parliament

When Parliament resumes next month, Senator Di Natale will ask the High Court to make a ruling on the two Greens who have quit the Upper House because they discovered they were dual citizens.

He challenged One Nation to clarify Senator Roberts' citizenship by asking the High Court to decide if he was eligible.

In the case of the two Greens, Scott Ludlam and Larissa Waters, the High Court will order a recount which will see them replaced in the Senate by the next Greens on the ticket.

Next month the Government will also refer the case of Nationals senator Matt Canavan to the High Court.

He has discovered he has dual citizenship and resigned from the ministry, but argues he should remain eligible to be in Parliament because he did not know his mother registered him when he was 25.

Senator Di Natale said it would be "decent and honourable" for One Nation to ask the High Court to also make a ruling about the cloud over Senator Roberts.

Cloud of confusion over citizenship and Senate eligibility

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 Di Natale also warned any MPs or senators that they would be defrauding the Commonwealth if they realise they are dual citizens and do not say anything.

"It is a very serious matter," he said.

"We have had senior legal advice that anyone who sits knowingly as a dual citizen being ineligible to stand potentially represents a fraud committed on the Commonwealth."

He said there should be an independent audit into the eligibility of all members of Parliament.

"It is a very straightforward test, it is laid out in our constitution what are the rules for eligibility," Senator Di Natale said.

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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.