Malcolm Roberts said he was confident he had been eligible for election, but had asked his anti-immigration, anti-Muslim party One Nation to refer him to the court "in the interests of honesty and transparency."
Roberts had been under intense media scrutiny before he revealed last month that he only received written confirmation that he was not a British citizen five months after he was elected in July last year. But he has declined to say whether he had ever been a dual citizen.
Roberts declined to say when he lodged a document renouncing British citizenship. He described this step as a precaution.
Roberts, who was born in the Indian city of Disergarh in 1955 to an Australian mother and Welsh father, follows senior government Sen. Matt Canavan and two minor Greens party senators, Scott Ludlam and Larissa Waters, in being referred to the court for scrutiny.
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New Zealand-born Ludlam and Waters, who was born in Canada, resigned last month after discovering they were citizens of their birth countries. But their party has asked to court to confirm that they were ineligible and to clarify rules around dual citizenship.
According to court precedent, Roberts was illegally elected if he was a British national on the July 2 election date.
One Nation party leader Pauline Hanson said she believes Roberts had been eligible to stand, but described his case as "very complex." She declined to elaborate.
Greens Leader Richard Di Natale said if One Nation had not asked the Senate to refer Roberts to the High Court, the Greens would have.
"His story has changed more times than I've changed underpants," Di Natale told the Senate.
Since the last election, a senator has quit because his indirect ownership of a building in which the government leased his office breached the constitution's eligibility rules because he was deemed to have had a pecuniary interest in the government contract.
The High Court fired another senator who had breached the rules twice by being bankrupt and having a criminal record for stealing the keys from a tow truck to prevent his truck from being repossessed three years ago.
"One might be forgiven for thinking that being a senator is one of the most hazardous occupations in Australia at the moment," Brandis told the Senate.