The five federal politicians deemed ineligible for office because they unwittingly held dual citizenship could be forced to pay back hundreds of thousands in salaries.

Barnaby Joyce, Fiona Nash, Scott Ludlam, Larissa Waters and Malcolm Roberts all earned six-figure salaries during their time in parliament.

And the five may be forced to pay it all back after they were officially booted from parliament by the High Court on Friday, though Ludlam and Waters had already resigned.

Barnaby Joyce could face the biggest bill. He has earned $473,987 since being sworn in as Deputy Prime Minister last year. His Nationals deputy Fiona Nash is not far behind on $398,843.

Greens senator Larissa Waters made $256,646 in the past 14 months.

One Nation's Malcolm Roberts and the Greens' Scott Ludlam are facing the smallest potential bills of $231,213.

It is up to the Department of Finance to send them a bill, but the government has the power to waive it.

If precedent is any indication, Special Minister of State Scott Ryan won't require the politicians to pay back their salaries.

Earlier this year Family First senator Bob Day was spared the debt after he was found to be invalidly elected.

Ryan stated Day had been serving in "good faith" and was entitled to his salary.

nine.com.au has reached out to the offices of Ryan and Opposition Leader Bill Shorten for comment.

Joyce is now running for his old job back, after his ousting forced a by-election in his electorate of New England, in northern NSW.

But his re-election could be complicated by another legal roadblock.

Labor has argued that any ministerial decisions made by Joyce could be challenged in court.

Section 64 of the constitution says "no minister…shall hold office for a longer period than three months unless he is or becomes a senator or a member of the House of Representatives".

The section can be read to mean any decision made by Joyce after October 20, 2016, could be voided.