Former One Nation senator Rodney Culleton has threatened a High Court challenge to his disqualification from federal Parliament for being bankrupt.
Senate President Stephen Parry informed West Australia Governor Kerry Sanderson of a vacancy in the upper house on Wednesday, brought about by the Federal Court's ruling on Mr Culleton's business affairs.
A 21-day stay on the bankruptcy order was due to end on Friday.
The small businessman and farmer quit Pauline Hanson's One Nation Party in December after a breakdown in relations with his colleagues, while facing a series of legal challenges including over his failure to pay a $280,000 debt to former Wesfarmers director Dick Lester.
Senator Parry advised party leaders and independent senators of the disqualification under section 45 of the constitution, which sees senate seats vacated if a senator is declared bankrupt.
He said the move did not end the question over the Senate's composition.
"The Senate has referred to the Court of Disputed Returns questions about the eligibility of Rod Culleton to have been chosen as a senator," he said in a statement.
"It will be necessary for the Court of Disputed Returns to deliver its answers to the referred questions before it will become apparent how the vacancy may be filled.
"The same situation applies to the vacancy in South Australia caused by the resignation of Bob Day whose eligibility to have been chosen as a senator was also the subject of a referral of questions by the Senate to the Court of Disputed Returns," Senator Parry said.
Both matters will be reported to the full Senate when it meets for the first time this year on February 7, while the High Court is due to return at the end of January.
A spokeswoman for Mr Culleton said he would not comment on Wednesday, but released his letter in response to Senator Parry.
It said the disqualification was "premature" and threatened legal action in a High Court application to be lodged on Thursday.
"I also inform you, President that there is an application to be filed in the High Court sitting as the Court of Disputed Returns tomorrow regarding another matter concerning yourself and that in the event you as the President refuse to withdraw the letter I may be left with no option but to join you in the High Court application," the letter said.
"This would be in the original jurisdiction of the High Court and within my rights. All my rights and remedies are reserved."
Mr Culleton has repeatedly denied being bankrupt and was preparing to lodge an appeal, but has also faced controversy after being convicted in absentia of a larceny charge.
Known for his colourful speeches and attacks against Australia's biggest banks, the 52-year-old was injured in a scuffle outside a Perth court earlier this month.
Last week he asked Senator Parry to recall the Senate to determine his status in Parliament.
The future of the now vacant Senate seat is unclear.
It could go to former One Nation's number two candidate for WA, Mr Culleton's brother-in-law Peter Georgiou, or a recount could take place.
One Nation would choose Mr Culleton's replacement for the spot if a casual vacancy is declared.
Senator Hanson did not respond to requests for comment, but took to social media.
"If a casual vacancy is declared because of Rod Culleton's disqualification I have already chosen a great person to replace him in the Senate," the Queensland senator wrote on Twitter.