As Senator Gallagher's dual citizenship case reaches a directions hearing at the court in Brisbane on Friday, ANU constitutional law professor Kim Rubenstein said she was likely to be cleared, granted she took all reasonable steps within her power to renounce her UK citizenship before nominating to run for the Senate.

That question - what amount to "reasonable steps" - is expected to be at the heart of the High Court's deliberations.

Senator Gallagher, the ACT's former chief minister and an up-and-coming interrogator at Senate estimates hearings, will have her case heard in the High Court after the dual citizenship saga claimed the scalps of multiple senators last year.

Professor Rubenstein said the High Court made clear in Queensland Nationals senator Matt Canavan's case last year it didn't believe another country's actions could necessarily impede the rights of candidates to represent their electorate or state.

"It would be a significant change in the court's interpretation to find her ineligible if she has done all that was required on her part, according to that other country's legislation, to renounce that citizenship," Professor Rubenstein said.

"It really is just a question whether on the day of nomination you've done everything within your power to renounce citizenship."

She said the High Court had been clear enough in its view on that issue and that it hadn't been necessary to refer Senator Gallagher's case.

Senator Gallagher has previously said she believed she had taken all reasonable steps to renounce British citizenship by descent from her father, but delays in processing her case by the UK Home Office meant she was a dual citizen at the time of nomination for the 2016 election.

A recount for her seat would likely follow a court decision finding her ineligible, and her three-year Senate term would probably go to Professionals Australia's ACT director David Smith, who would receive the ALP's above the line votes at the 2016 election.

University of Sydney constitutional law professor Anne Twomey said the case will centre on whether a "reasonable steps" test applies when it is possible to renounce foreign citizenship.

She said the court will also consider whether it is enough that the candidate took all steps within their power before the nomination date, or whether these have to be completed in the foreign country before that date, or undertaken at a reasonable period before to allow for their completion in time.

Justin Gleeson SC, who left his role as solicitor-general following a public deterioration in relations with ex-attorney-general George Brandis, will argue the case for Senator Gallagher after also acting for Tony Windsor in Nationals leader Barnaby Joyce's High Court dual citizenship case last year.

Labor has enlisted Maurice Blackburn as Senator Gallagher's solicitors.

Legal advice from British and Australian barristers says she took every reasonable step to renounce citizenship by descent from her English-born father Charles Gallagher before nominating, and that she meets the standard set by the High Court in recent section 44 cases.

The Senate voted to refer Senator Gallagher to the High Court over her dual British citizenship in December, when she also stood aside from her responsibilities on Labor's frontbench until her case was resolved.

Batman Labor MP David Feeney's case will also go to the High Court for a directions hearing on Friday after he admitted he could not find documents proving he renounced his British citizenship.

Mr Feeney will be represented by Arnold Bloch Leibler and Noel Hutley SC.