A student who has cerebral palsy, a man who only got 19 votes, and a veteran Greens campaigner will replace three of the four senators caught up in the citizenship purge.

The fourth hopeful, a woman at the centre of a bitter factional feud in her own party, faces a High Court hearing before she can take her seat.

The Senate spots occupied by Fiona Nash (NSW), Scott Ludlam (WA), Larissa Waters (Queensland) and Malcolm Roberts (Queensland) were made vacant when the four senators were found ineligible to serve in Parliament due to their dual citizenship.

Former deputy prime minister Barnaby Joyce was also booted from the Parliament, and is facing a by-election for his Lower House seat.

Section 44 of the constitution prohibits dual citizens from serving in federal parliament.

Legal precedent requires a Senate recount in situations where sitting members of parliament are found to be in breach of section 44.

There is a complication over the replacement for Ms Nash, because NSW Liberal Hollie Hughes held a taxpayer-funded job at on the Administrative Appeals Tribunal between losing the election and the Senate recount.

Another section of the constitution bans people from holding parliamentary office if they also held an "office of profit under the crown".

The matter will be dealt with by the High Court next Wednesday.

Who are the new senators?

Hollie Hughes was the next candidate on the shared Liberals-Nationals New South Wales ticket after former deputy Nationals leader Ms Nash.

Hollie Hughes was bumped to sixth spot on the ticket in last year's election. ( Supplied: NSW Liberal Party )

A powerful member of the Liberal Party executive, she missed out at last year's election after Malcolm Turnbull called a double-dissolution election — meaning all 12 Senate spots would be up for grabs — saw her moved to the sixth spot on the Liberal-National ticket.

That effectively put her in an unwinnable position behind Defence Minister Marise Payne, Cabinet secretary Arthur Sinodinos, former senator Ms Nash, Liberal senator Concetta Fierravanti-Wells and Nationals senator John Williams.

At the time, she said she was "disappointed" and remained hopeful six senators could be elected.

The New South Wales Liberal Party is factionally divided and that split has already had an impact on Ms Hughes.

Some Nationals had been hopeful Ms Hughes would take up Ms Nash's vacant Senate seat and then resign, creating a casual vacancy Ms Nash could step back in and fill.

Ms Hughes had been appointed by the Turnbull Government this year to sit on the Administrative Appeals Tribunal, which reviews government decisions. It is a taxpayer-funded job, and that is the reason there are questions over her eligibility.

Queensland has received two new senators, one each from the Greens and One Nation.

Replacing co-deputy Greens leader Larissa Waters is another longtime senator, Andrew Bartlett.

Former Australian Democrats senator Andrew Bartlett joined the Greens in 2009. ( Facebook: Andrew Bartlett )

Mr Bartlett spent 11 years in the Senate with the Democrats, including two as the leader, but left the job when the Democrats collapsed in 2008.

He joined the Greens the following year and continued campaigning on animal rights, refugee issues and disarmament.

Mr Bartlett is not expected to serve long in the Senate as it is likely he will stand aside potentially early next year so Ms Waters can take his place.

One Nation's Malcolm Roberts will be replaced by Fraser Anning.

Fraser Anning will serve the remaining two years of Malcolm Roberts' three-year term. ( Supplied: One Nation )

Mr Anning was third on One Nation's Queensland Senate ticket and received 19 first preference votes at last year's poll.

For a time, there was some doubt about whether the long-time supporter of Senator Hanson would be eligible for Parliament, as he was facing bankruptcy legal action.

Mr Anning and his wife owed more than $212,000 to Bendigo and Adelaide Bank, but the case was withdrawn last month.

Western Australian Greens senator Scott Ludlam, whose dual citizenship discovery sparked the entire saga, will be replaced by the youngest senator to date, Jordon Steele-John.

Jordon Steele-John first ran in a federal election in 2013, when he was 18. ( Instagram: thegreenswa )

Mr Steele-John is a 23-year-old university student and a disability advocate who has cerebral palsy and uses a wheelchair.

He first ran in a federal election as the Greens' candidate for Fremantle in 2013 when he was 18.

Mr Steele-John has rights for those with a disability and youth unemployment at the top of his agenda.