After initially indicating it, too, would support referring Senator Roberts, the government, which needs the support of One Nation in the Senate, has backed of.

On Tuesday, the Senate voted to refer to the High Court former Greens Senators Larissa Waters and Scott Ludlam, and Nationals Senator Matt Canavan.

Mr Ludlam and Ms Waters quit the Senate over the winter break after learning they held dual citizenship. Mr Ludlam was a dual New Zealand citizen and Ms Waters was a dual Canadian citizen.

The High Court is expected to order a recount under which they will be replaced by Greens Senators who were next on the ticket in their respective states of Western Australia and Queensland.

However, Senator Waters announced via Twitter on Tuesday she was no longer a Canadian citizen and that she would recontest her seat at the next election.

"I'm no longer Canadian! & I have more to do in the Senate, so I'll nominate to recontest. In meantime we @QldGreens have state seats to win!" she tweeted.

She is due to be replaced by former Democrats Senator Andrew Bartlett, who has since joined the Greens. But it is possible Ms Waters could be back sooner.

Sources said Mr Bartlett could step aside after the Queensland state election, due within months, enabling Ms Waters to fill the casual vacancy, or step aside at the next federal election handing Ms Waters the number one spot on the Greens Queensland Senate ticket.

Senator Canavan resigned from cabinet but not the Senate. He will argue in the High Court that is Italian citizenship is invalid under Italian law because he was registered up by his mother without his knowledge and, as an adult at the time, he was required to fill out and sign the necessary paperwork himself. He told the Senate he will not vote in Parliament until his case is resolved.

Senator Roberts only received formal notice in December last year - almost six months after the election - that his British citizenship was renounced. His only effort before the election was to send the High Commission a handful of emails in the weeks before polling day to inquire if he was a British citizen and, if he was, that he renounced it.