The path has been cleared for the High Court to endorse a new candidate to replace former Nationals senator Fiona Nash, after the court explained why it rejected Liberal Hollie Hughes.

Ms Hughes was nominated after a special countback of ballot papers from the 2016 federal election when Ms Nash lost her seat due to her Scottish heritage.

But a unanimous decision from the court found she was not eligible, because she had taken up a government job in the interim.

Section 44 of the constitution prevents people who are dual citizens from serving, but also prohibits people who have a government job from running for Parliament.

Ms Hughes resigned from her position on the Administrative Appeals Tribunal (AAT) 45 minutes after the finding against Ms Nash.

But the court has found it was always a possibility Ms Hughes might be chosen to fill a vacancy, and by taking a government job in the meantime she lost her chance.

A countback would likely see Liberal and former Army officer Jim Molan selected.

'Voluntary step' led to disqualification

The main issue examined by the court was the timing of Ms Hughes' appointment to the AAT and the subsequent recount.

"The issue was whether holding that disqualifying office during the discrete period between 1 July and 27 October was enough to render Ms Hughes incapable of being chosen," the judgment said.

The unanimous finding was delivered by Chief Justice Susan Kiefel and Justices Virginia Bell, Stephen Gageler, Patrick Keane and James Edelman.

They acknowledged the decision could seem "harsh or unduly technical" but said it needed to be understood in context.

"Ms Hughes' acceptance in the meantime of appointment to the Administrative Appeals Tribunal, with the entitlement to remuneration which that appointment brought, was understandable," the judgment said.

"But it was a voluntary step which she took in circumstances where ... disqualification or lack of qualification of a senator who had been returned as elected was always a possibility.