There are growing calls by the Greens for an audit of all members of parliament as the Senate President revealed he could be a British citizen.
Senator Stephen Parry said that after the unanimous ruling handed down by the High Court last Friday, which disqualified five parliamentarians, he had examined his citizenship status.
On Tuesday, he said he would quit parliament if British authorities confirmed he was a dual citizen through his UK-born father.
His father moved to Australia as a boy in 1951 and he was born in Tasmania in 1960.
He has written to the British Home Office seeking clarity about his UK status and expects to hear back within days.
If found to have UK ties he will quit as Senate president and senator for Tasmania, rather than wait for the High Court to decide - as it did with seven other members of parliament.
Senator George Brandis doesn't believe there is anyone else in the same position.
But he expressed the same confidence on Sunday, just a day before Senator Parry contacted him.
A Liberal source said: "There could be dozens more and not just on our side."
Labor and the government have resisted the calls for an audit of all members of parliament.
The prime minister has sought advice on whether citizenship or electoral laws could be changed to head off the problem in the future, without the need of a referendum to change section 44 of the constitution.
Former senator Richard Colbeck would likely get the seat if Senator Parry is disqualified and a special count is ordered by the court.
If it is declared a casual vacancy through resignation, the Tasmanian Liberals would decide.
Possible Senate presidential replacements include Liberal senators Dean Smith, David Bushby and David Fawcett.