Ms Nash, the former deputy leader of the federal Nationals, was disqualified under section 44 (i) of the constitution and her place was set to be filled by Liberal Hollie Hughes, who was the next person on the joint Coalition senate ticket.

But Ms Hughes faces constitutional issues of her own, as she was recently appointed to the Administrative Appeals Tribunal by the Turnbull government, which could disqualify her under section 44 (iv) of the constitution for holding an office of profit under the crown - even though she had no conflict at the time of the July 2, 2016 poll.

On Friday, Justice Stephen Gageler referred Ms Hughes confirmation to a full bench hearing next Wednesday after advising that she may be constitutionally ineligible to sit in the Senate.

Fairfax Media can reveal the Nationals have received confidential advice from high-profile barrister Bret Walker SC about Ms Hughes' eligibility about 10 days ago, and the likelihood of Ms Nash being able to resume her seat if Ms Hughes is disqualified by the High Court.

Nationals contacted by Fairfax Media said they were hopeful the popular Ms Nash could be returned to Parliament, though they acknowledged she faced an uphill battle.

Former Australian Army general and Liberal candidate Jim Molan is the next person on the NSW senate ticket after Ms Hughes and had been considered the front runner to take her spot if the High Court rules she cannot take the seat.

Colleagues of Mr Molan, however, said he was not planning to make way for Ms Nash and would put his hand up for the Senate seat if it becomes available - setting up a potential clash between the Coalition parties.

But according to constitutional law professor George Williams, as Ms Hughes was a valid candidate at the time of the July 2, 2016 election, if the High Court rules her out then a casual vacancy would be created and that could be filled by Ms Nash.

"In her [Ms Hughes'] case it would be that anyone could fill the vacancy from her party. It's even possible Ms Hughes could fill her own casual vacancy," he said.

The constitution clearly states that a casual vacancy must be filled by a person from the same political party.

Ms Hughes, as a Liberal, and Ms Nash, as a National, belong to different parties, which could be an impediment to the former deputy's return to Parliament via the casual vacancy.

However, the Liberals and Nationals run on a joint Senate ticket in NSW, which could offer a lifeline to Ms Nash - possibly with the assistance of a behind the scenes deal between party headquarters.

On the question of whether Ms Hughes would be found to have been not validly elected, Professor Williams said her situation was not promising.

"On the face of it she is disqualified, as she held an office of profit between the time of nomination and her selection," he said.

"It does seem quite unfair. The question is whether the High Court will fashion an exception for her, and it is quite possible they will."

The NSW Liberal party has received its own legal advice, which senior party sources say left them "confident" Ms Hughes was eligible because her appointment post-dated the election.

Constitutional expert Professor Graeme Orr from the University of Queensland law school said that it was unlikely the appointment would cause Ms Hughes to be found ineligible.

"Ms Hughes did not somehow remain a 'candidate' for the last year," he said. "The 2016 election ended with the return of the writ last year.

"It is reopened for the limited purpose of filling the seat, using a recount of 2016 votes. But only to fill the seat for the future. Ms Hughes does not magically become a senator as of 2016. The High Court is not a time machine."

Prime Minister Malcolm Turnbull said on Friday that given Ms Hughes' case had been referred to the full bench of the High Court, "I will leave it to the court to determine. Once again, they are the only ones who can make these determinations."

Mr Turnbull also brushed aside questions about NSW MP John Alexander, who Fairfax Media revealed earlier this week could be a dual citizen as his father, Gilbert Alexander, was born in the UK and may have conferred citizenship by descent on the former tennis champion.

Speaking from Vietnam where he is attending APEC, he said he had not heard from Mr Alexander in recent days.

"I understand the position is as when I last heard from him, which is, he said this publicly, he believes he is not a dual citizen," he said.

Labor deputy leader Tanya Plibersek said on Friday that Mr Alexander had gone to ground and that if was a dual citizen, he should step aside immediately.

The High Court on Friday gave the green light to three new senators to fill vacancies created by other MPs struck down by dual citizenship laws.

Former Australian Democrats leader Andrew Bartlett took former Greens senator Larissa Waters' place in Queensland; Fraser Anning will take the Queensland One Nation seat vacated by Malcolm Roberts and Jordon Steele-John will replace Scott Ludlam as Greens senator for Western Australia.

Comment was sought from the NSW Nationals and from Ms Nash, Mr Molan and Ms Hughes.