A Sunshine Coast man and his legal team have made history by being the first to successfully use a little-known law to sue a sitting politician — One Nation senator Malcolm Roberts..

The action was taken over Mr Roberts' eligibility to sit in Parliament because of his dual citizenship.

Key points: The High Court has ordered One Nation senator Malcolm Roberts to pay $6,000 plus legal costs to Queensland political blogger Tony Magrathea

The High Court has ordered One Nation senator Malcolm Roberts to pay $6,000 plus legal costs to Queensland political blogger Tony Magrathea Mr Magrathea sued Mr Roberts over his right to sit in parliament in 2016 due to his dual citizenship

Mr Magrathea sued Mr Roberts over his right to sit in parliament in 2016 due to his dual citizenship It marked a landmark first successful-known use of the little-known Common Informers Act

Tony Magrathea said he first alerted Senator Roberts to the possibility of his dual citizenship after the 2016 election.

"He ignored me and blocked me on social media," Mr Magrathea said.

After issuing a similar warning to Senator Roberts' wife on Facebook, Mr Magrathea said the matter was named in the High Court chronology and this then prompted him to use the little-known Common Informers Act to challenge Senator Roberts over his citizenship status.

One Nation leader Pauline Hanson pictured with Malcolm Roberts. ( AAP Image: Dan Peled )

David vs Goliath legal battle

With distinct echoes of the Australian movie The Castle, where character Darryl Kerrigan goes to the High Court on constitutional grounds to fight a plan to acquire his home for the local airport expansion, Mr Magrathea took on what could be described as a David and Goliath battle.

In contrast to The Castle plot though, where Kerrigan initially chooses an incompetent lawyer — Dennis Denuto to represent him, Tony Magrathea ran his plan by local barrister Clem van der Weegen, who cautiously agreed to take on the case on a pro bono basis, if Mr Magrathea could find a solicitor, which he did, enlisting Rowena Ferrall.

Barrister Clem van der Weegen acted for blogger Tony Magrathea in his successful case against One Nation senator Malcolm Roberts for sitting while ineligible. ( ABC Sunshine Coast: Megan Kinninment )

Law 'dates back to Middle Ages'

Mr Van der Weegen explained the rich historical nature of the Common Informers Act which he says goes back to the Middle Ages.

"Before police forces were in existence, the king used to ask his subjects to go out and enforce the law," he said.

"And in return, they would then get the penalty for their troubles.

One Nation Senator Malcolm Roberts returned to the Senate in 2019 after his election had previously been declared invalid by the high court in 2017. ( News Video )

"So this is a very ancient concept that has its remnant form in the Common Informers Act, where citizens can go and sue for the penalty if the High Court declares that they're [a sitting parliamentarian is] incapable of being chosen or sitting in parliament."

The penalty is calculated at $200 plus $200 for every day that the Senator sat in parliament while ineligible, which was 29 days in total.

The High Court finding was passed down on June 24.

Senator Roberts was returned to the Senate in 2019, after his earlier 2017 election had been declared invalid.

Setting a 'historical precedent'

While Mr Magrathea has been awarded more than $6,000 by the High Court as the penalty incurred by Senator Roberts, he said money was not his motive for bringing the legal action.

"The big thing was getting this historical precedent, where I'm the first and my solicitor and barrister are the first to have ever done this," he said.

Mr Van der Weegen said he was in negotiation with Senator Roberts' lawyers over court-ordered costs.

"It's not cheap to go to the High Court, so it's something that is yet to be determined," he said.

Senator Roberts said he did not contest the matter in court.

"Mr Magrathea's lawyer sent me a letter in 2017," he said.

"I immediately complied with the Constitution's provisions.

"We then waited many, many months for the High Court to seal the agreement, as it did recently."

Editor's note 18/07/2019: This article has been updated from its original publication date of 17/07/2019 to clarify that the law was used for the first time on a sitting politician.