The High Court will decide whether a Liberal candidate who took on a tribunal role after the 2016 federal election can replace a disqualified minister in the Senate.
Hollie Hughes was the next candidate down the NSW coalition Senate ticket behind then Nationals deputy leader Fiona Nash at the election.
Ms Nash was disqualified by the High Court on October 27 because she held dual citizenship, which breached section 44 of the constitution.
A court-ordered special count resulted in Ms Hughes being elected to the Senate seat.
However there is doubt over Ms Hughes' eligibility given that she took up a part-time role with the Administrative Appeals Tribunal on July 1 this year.
The government admits the AAT job is an "office of profit under the Crown", which would also disqualify a person from parliament.
But Solicitor-General Stephen Donaghue will argue Ms Hughes should be duly elected because the process of "being chosen" started with nomination and concluded at the end of polling day.
The special count of ballots which resulted in Ms Hughes' election was "not a re-choosing but simply the proper identification of the electorate's choice made on polling day", he will argue.
Ms Hughes' barrister Arthur Moses SC argued in his written submission the votes which led to her taking the Senate seat were cast at the time of the 2016 election, when she was qualified to be in parliament.
However the court will also hear arguments from amicus curiae Geoffrey Kennett SC, who says Ms Hughes was incapable of being chosen because she held the AAT position between September 4-5, when the Senate resolved to refer the Nash case to the court, and October 27.
