Justin Gleeson SC, who left his role as solicitor-general following a public deterioration in relations with Senator Brandis, will argue the case for Senator Gallagher after she was caught up in the dual citizenship fiasco that has ejected multiple parliamentarians.

Mr Gleeson also acted for Tony Windsor in the High Court, fighting the former New England MP's nemesis, deputy Prime Minister Barnaby Joyce, as the Nationals leader's citizenship case was heard in October.

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.

A finding on her eligibility could have implications for other Labor MPs under a cloud over dual citizenship claims, but the ALP says it won't consider a High Court ruling on Senator Gallagher as a precedent.

Senator Gallagher 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.

Documents provided to the Senate this month showed she was "at the date of her nomination for the 2016 election, a British citizen by descent" and that her moves to renounce in April 2016 took until August 16 to be completed by UK officials.

Labor has enlisted Maurice Blackburn as Senator Gallagher's solicitors and it is understood the High Court will decide whether the ALP or the government bears the costs for her legal representation upon making a ruling.

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

Mr Gleeson argued in Mr Joyce's High Court case the Nationals leader should be disqualified as an MP, warning the High Court that chaos and uncertainty accompanying Parliament's dual citizenship crisis will be repeated in future if the court accepted that ignorance was a valid defence.

Section 44 of the constitution required strict "undivided loyalty" from MPs and dual citizenship did not need to be "voluntary, chosen or felt" to create a split allegiance, Mr Gleeson said.

Clearing Mr Joyce and other MPs on the basis that they were ignorant of their status would set a dangerous precedent, he said.

The High Court will hold a directions hearing for Senator Gallagher's case on January 19 in Brisbane.