Ms Hughes was set to fill the Senate seat left vacant after former Nationals deputy leader Fiona Nash was found to be ineligible under section 44 (i) of the constitution.

But Justice Stephen Gageler referred Ms Hughes confirmation to a full bench hearing next Wednesday after hearing that Ms Hughes may be constitutionally ineligible to sit in the Senate because she was a Commonwealth employee for a time after the 2016 election.

The High Court on Friday did give 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 will return to the upper house, replacing Larissa Waters as Greens senator for Queensland.

Business consultant Fraser Anning will step into the Queensland One Nation seat vacated by Malcolm Roberts.

And 23-year-old student Jordon Steele-John will replace Scott Ludlam as Greens senator for Western Australia.

Questions were raised about the eligibility of the Ms Hughes due to her recent appointment to the Administrative Appeals Tribunal. MPs are forbidden from holding "any office of profit under the Crown" under section 44 (iv) of the constitution.

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."

If Ms Hughes is found to not be eligible for the vacant Senate post, the spot could be filled by former general Jim Molan, who occupied the seventh spot on the Coalition's senate ticket.

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.

Mr Alexander has sought advice from UK authorities about his status but is yet to report back their findings.

The Prime Minister, speaking from Vietnam where he is attending APEC, 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.

"If you are satisfied that you are not constitutionally eligible to sit in the Parliament, you shouldn't be sitting. That is fundamental. I recognise there are cases that will be line ball decisions or questions of doubt where legal opinions will differ and those of the cases that should be referred to the High Court."

But 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.

"Where is John Alexander's answer and why doesn't Malcolm Turnbull make him stand up and confess whether he is a dual citizen or not?"

"The Prime Minister is also trying to push this off until Christmas or January. Labor says people should be able to get their paperwork in order by the 1 December. We have been debating this now for weeks, for months."

The Coalition wants Labor MPs such as Labor's Justine Keay, Josh Wilson and possibly Susan Lamb - who did not receive formal notification of renunciation of their UK citizenship until after nominations for the 2016 election closed, possibly putting them in breach of section 44 (i) - referred to the High Court. NXT MP Rebekha Sharkie faces a similar problem.

Labor is stonewalling on referring these MPs to the High Court.

Mr Turnbull has also threatened to break with longstanding precedent and use the government's slim majority to refer any Labor MPs under a citizenship cloud to the High Court.

The Prime Minister and Opposition Leader Bill Shorten are yet to agree on a mechanism on how to bring the citizenship crisis to a head.

The three confirmed senators will be sworn in on Monday ahead of the Senate voting on a new president to replace Liberal Stephen Parry, who resigned over his dual citizenship.

The position is tipped to go to Special Minister of State Scott Ryan.

- with AAP