A replacement for dumped senator Rod Culleton remains unknown but it could be the second person on the One Nation ticket - his brother-in-law Peter Georgiou.
The independent senator lost his spot on Wednesday after he was declared bankrupt by a Federal Court judge.
Senate president Stephen Parry said in a statement that he had informed Mr Culleton and WA governor Kerry Sanderson there was a vacancy because the former One Nation member had been disqualified from the position as he was an "undischarged bankrupt".
Senator Parry also advised party leaders and independent senators of the development.
But the Court of Disputed Returns is yet to determine whether Mr Culleton was ever eligible to be selected as a senator as he had been convicted of a larceny charge, which was later annulled, at the time of the federal election in July.
"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," Senator Parry said.
It will be reported to the Senate when it meets on February 7.
Depending on the court's decision, the vacancy will be filled by either Mr Georgiou, or the party can nominate a replacement.
"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," One Nation leader Pauline Hanson tweeted on Wednesday night.
Mr Culleton had been ordered to pay former Wesfarmers director Dick Lester $205,536 in 2013 - which has since increased to $280,000 - over unpaid rent relating to a $13.2 million 2009 property acquisition deal that soured.
He has repeatedly denied he is bankrupt.
Mr Culleton was elected in the July federal election as a representative of Pauline Hanson's One Nation but resigned in December from the party.