The High Court will be asked to determine whether One Nation senator Malcolm Roberts is eligible to sit in parliament.
The Greens have won over key crossbench votes to refer the Queensland senator to the Court of Disputed Returns, a day after the upper house referred the cases of Nationals senator Matt Canavan and two former Greens senators over their dual citizenship.
The minor party believes the eligibility of Senator Roberts could be in doubt because his British citizenship disqualified him from contesting the 2016 election.
The Greens have garnered the support of independent senator Derryn Hinch and the three Nick Xenophon Team senators.
Senator Hinch said he spent a long time discussing the issue with Senator Roberts in his office on Tuesday but was not convinced.
"I think yesterday Senator Roberts was economical with the truth when talking to me," he told reporters.
Senator Xenophon said his team would also back the referral.
"Because that is a fair thing to do," he said, adding his team also supported an independent citizenship audit of all MPs.
Liberal Democrat David Leyonhjelm said he was on good terms with One Nation senators and was unsure whether he would support the referral.
"It's not a black and white issue, it's a matter of grey. I'll listen to the debate," he said.
"If it isn't referred it'll leave question marks over his eligibility and it'll probably end up being decided by the High Court anyway."
Senator Roberts and One Nation leader Pauline Hanson have repeatedly denied he ever held British citizenship.
But in an immigration department document signed by Senator Roberts and dated May 8, 1974, the then 19-year-old notes he is a "British UK and Cols (Colonies)" citizen.
The Queensland senator was born in Disergarh, India, in 1955 and his family arrived in Australia in October 1962.
While the documents confirm the Roberts family's notifications to become citizens were approved by the immigration department, there is no mention that the British citizenship was renounced.
Senator Roberts insists he took steps to renounce any "possible" British citizenship before he ran for parliament, but did not receive a letter back from British authorities until December last year - five months after the election.
The federal government argues there isn't enough evidence to back the referral.
"It is a very serious matter for a group of politicians sitting in a parliamentary chamber to decide to effectively disqualify or put a question mark over the eligibility of one of their number," Attorney-General George Brandis said.
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