After a protracted legal battle that began almost as soon as he was elected, Mr Culleton's senate election has been ruled invalid by the High Court.

On Friday, the unanimous full bench decision of the High Court, sitting as the Court of Disputed Returns, puts an end to the former One Nation senator's battle to stay in the upper house.

Mr Culleton was charged by police from the Armidale Target Action Group after stealing the tow truck driver’s key during an altercation in April, 2014, at his Guyra base.

But when he failed to front up to court, Magistrate Michael Holmes convicted him in his absence – a conviction that stood when he was elected as a senator for One Nation in Western Australia during the July federal election.

Culleton – who was arrested after a warrant was issued by the court – tried to have the conviction overturned and pleaded not guilty in Armidale Local Court.

In the end, the conviction was annulled by Mr Holmes who said it was a trivial offence over a key but Culleton acted “foolishly”.

Culleton told The Leader outside court he was acting in “self-defence” when the key was flung during the confrontation.

“I just basically can say some days dogshit does stick to your boot but I can say I want to go on from here,” he said in Armidale on October 25.

The Senate referred his election to the High Court given that, at the time he was elected, he was potentially ineligible under the constitution.

Mr Culleton's legal team argued that, given the conviction was annulled, it had never existed and therefore did not affect his eligibility.

But the court found that, at the time of the July 2016 election, Mr Culleton had been convicted and was subject to a term of imprisonment of more than a year "both as a matter of fact, and as a matter of law" and the later annulment "had no effect on that state of affairs".

Mr Culleton was not in court for the judgment on Friday.

Parliament will not know who takes his place until after a recount. The Electoral Commission will now consider the ballots as if Mr Culleton's name had not existed on the ticket.

His brother-in-law, Peter Georgiou, who was the number two candidate for One Nation at the July 2 election, is considered most likely to fill the vacancy. Mr Culleton has previously vowed to return to Parliament House as his chief of staff.

Mr Culleton quit One Nation in early December, amid reports of a growing rift with party leader, Pauline Hanson. The rift emerged after Senator Hanson swung her party's support behind a government motion referring Mr Culleton's election to the High Court.

The High Court judges found that Mr Culleton was not eligible to stand for the Senate at the time of his election, because he had been convicted and was subject to be sentenced for an offence that carried a year or more imprisonment.

That related to a larceny charge from New South Wales over a 2014 dispute with a tow truck driverwho arrived to repossess property from Mr Culleton's property, resulting in the loss of what Mr Culleton referred to as "a $7.50 key".

Mr Culleton was convicted in his absence by the Armidale Local Court in March 2016, but in August, following the election, won his bid to have the conviction annulled.

The Senate referred his election to the High Court given that, at the time he was elected, he was potentially ineligible under the constitution.

Mr Culleton's legal team argued that, given the conviction was annulled, it had never existed and therefore did not affect his eligibility.

But the court found that, at the time of the July 2016 election, Mr Culleton had been convicted and was subject to a term of imprisonment of more than a year "both as a matter of fact, and as a matter of law" and the later annulment "had no effect on that state of affairs".

A Federal Court ruling into Mr Culleton's bankruptcy is due to be handed down late on Friday, but will have no impact on his place in the Senate, as the High Court ruling is final.

He accused the party of "un-Australian behaviour" and having betrayed their election promises.

Senator Hanson described her former senator as a "pain in my backside" and said she was happy to see him leave the party.

The court ordered the Commonwealth to pay Mr Culleton's legal costs.

While the Senate vacancy still remains subject to a recount, One Nation's showing in the July election has given Senator Hanson confidence to welcome Mr Georgiou "to the team", releasing a statement 

calling him "an asset" for the Senate and someone who "will serve the people of Western Australia with distinction and dedication", while taking one last swipe at Mr Culleton.

"After meeting Peter and getting to know him, it is clear he is a dedicated, hardworking man who believes strongly in the principles of One Nation," she said.

"...Peter's not a career politician, he has real world experience and real world insight that will be invaluable to the party and the to the people of Australia on the floor of the Senate.

"...One Nation is united in our excitement at the prospect of working with Peter Georgiou and, honestly, I couldn't be happier.  It looks like One Nation and Western Australia will be getting a significant senator upgrade."