The High Court has found the election of former One Nation Senator Rod Culleton was invalid at the last election.
The court has ordered the vacancy be filled by a special recount of the WA senate vote with details to be determined by a single justice.
The court was asked to rule on whether a conviction at the time of the poll was grounds for disqualifications even though it was later annulled.
The Senate referred the validity of Mr Culleton’s election in November, claiming he had a conviction against his name at the time of the poll for stealing the keys to a tow truck.
Under the Constitution, that would disqualify him from sitting in Federal Parliament.
Mr Culleton had argued he was in the clear, because the conviction was annulled after the election on July 2 and after he was declared the 12th senator for WA.
However, the Commonwealth maintained that at all times during the election process he was disqualified from running for office.
In December, the Federal Court ruled he was an “undischarged bankrupt” in a case brought by former business partners — another ground for disqualification from the Senate.
Mr Culleton appealed that ruling on several grounds, including that he was solvent when the finding was made and that the matter should have gone before a jury.
His appeal was also thrown out of court on Friday,
Earlier this year, Senate president Stephen Parry followed that disqualification and declared Mr Culleton’s seat was vacant.
Mr Culleton quit Pauline Hanson’s One Nation party during the proceedings, after deep divisions emerged between the West Australian and his party leader.
He voted against the party on a number of policies, including the backpacker tax.
Heated arguments between Senator Hanson and Mr Culleton’s offices also came to light, with accusations of bullying and a breakdown in communication.