Retired Army officer Jim Molan has been declared a senator for New South Wales, replacing former Nationals deputy leader Fiona Nash, who was forced out of Parliament because of her dual citizenship.

The former Major General and Iraq war veteran, who helped develop the Coalition's border protection policy, including boat turn backs, was the winner of a special count of votes held last month.

Liberal Hollie Hughes was to be senator Nash's replacement, but the High Court refused to endorse her because she took up a government job with the Administrative Appeals Tribunal after the election.

A protracted hearing in the High Court prevented Mr Molan from taking up his seat for the Liberals.

The sticking point was whether Mr Molan should receive a six, or three-year term in Parliament.

The ABC's election analyst Antony Green said that was because the senators had to be allocated to short and long terms at the last election.

Fiona Nash and Hollie Hughes were both ineligible, but for different reasons. ( AAP/Supplied )

"Because the last Senate election was a double-dissolution election, the senators had different terms," he said.

"Fiona Nash's term was six years because she was one of the first six candidates elected.

"However, because Jim Molan was seventh on the ticket, there is conjecture as to whether he should receive a six, or three-year term."

At a directions hearing in Sydney, Commonwealth solicitor-general Stephen Donaghue QC said it was "in the public interest that Mr Molan be able to take his seat" before Parliament resumes in February.

Mr Donaghue said there was no dispute about the outcome of the special count of votes.

Geoffrey Kennett SC, an independent barrister appointed to assist the court, said the question of the length of terms should be "resolved rather than disappearing".

But Justice Stephen Gageler said: "There should be no lingering uncertainty and it is in the public interest that the issue be resolved as soon as possible."

He declared "Jim Molan is elected a senator to the state of New South Wales".

Mr Molan said he would continue to push for changes to the NSW Liberal Party's preselection policy.

"I am on record in relation to my views on democratic reform, and I'm not going to change that now that I find myself in the Senate," he said.

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"My hope is that members of the New South Wales division will determine all preselection from now on and this will be considered on the 10th of February at the annual general meeting.

"This is all about party members being able to exercise democracy through plebiscites for everything from the lower, upper, state executive and committees."

Mr Molan received more than 10,000 first preference votes at the last election.

He was the seventh candidate on the Liberals' and Nationals' joint ticket.

The changes to the Coalition's senate team have implications for how long senators Concetta Fierrvanti-Wells and John Williams will be entitled to sit in Parliament.

The solicitor-general said there would be "no impediment" to the Senate referring the question of terms to the full court of the High Court.