It is expected Justin Gleeson SC will appear in the High Court next month acting for Mr Windsor, who is party to the proceedings because he was the runner-up for Mr Joyce's seat of New England at the 2016 election.

Mr Gleeson will do battle with his successor as solicitor-general, Stephen Donaghue, who is representing the Commonwealth in its defence of Mr Joyce and government MPs Matt Canavan and Fiona Nash - and by extension senator Nick Xenophon.

Fairfax Media understands Mr Gleeson will share barrister duties with Ron Merkel QC, who is assisting Mr Windsor in the case along with the Fitzroy Legal Centre. Taxpayers are footing the bill the legal defence of all the MPs caught in the citizenship net, and of Mr Windsor.

Mr Windsor confirmed Mr Gleeson had joined his team but did not want to comment on details.

"I'm not getting into who's doing what. There's a good team of people there," he told Fairfax Media on Friday.

"They're scrutinising the issues as they understand it in terms of the law and I've got faith in them and I've got faith in the court itself."

Mr Gleeson resigned as the government's lawyer in October 2016 following a protracted breakdown in his relationship with Senator Brandis.

In his resignation letter, he said the relationship was "irretrievably broken" and lamented the lack of trust, confidence and respect between himself and the Attorney-General.

Mr Gleeson staunchly rejected "each and every attack and insinuation that has been made in recent times upon me personally, upon my office, by government members of Parliament, including you".

At the heart of the stoush was Senator Brandis' legal directive, later overturned, that prevented ministers seeking legal advice from Mr Gleeson without the approval of the Attorney-General.

Fairfax Media understands Mr Gleeson's new role on the Windsor legal team was met with amusement within Senator Brandis' office, and a view the former solicitor-general was "out for revenge".

Senator Brandis declined to comment. Mr Gleeson was overseas and could not be reached.

Constitutional expert and UNSW Dean of Law George Williams, who has links to Labor, said the legal Clash of the Titans would be one to watch.

"It makes perfect sense that Tony WIndsor would brief someone of his [Mr Gleeson's] stature and quality," he said.

"There's no doubting his expertise and legal ability, and of course his experience. When the stakes are very high you want the best.

"It certainly bears watching to see how a former solicitor-general takes on the Commonwealth's counsel in such a major case."

The High Court last week agreed with the government's intention to treat the cases of Mr Joyce and senators Nash, Canavan and Xenophon under the same legal argument as they were "materially indistinguishable".

Mr Donaghue submitted it was "almost inevitable" those four cases would "stand or fall together", although the facts of each matter may differ.

Prime Minister Malcolm Turnbull has expressed a high degree of confidence in Mr Donaghue's advice that Mr Joyce will be ruled eligible to sit in Parliament. Neither Mr Joyce nor Senator Nash have stood down from cabinet in the interim.