A Liberal candidate should be able to replace former cabinet minister Fiona Nash immediately even if someone wanted to challenge the decision in the future, the High Court has heard.
The court is considering, in the first case of its kind, whether Hollie Hughes should be declared duly elected to replace Ms Nash as a NSW coalition senator.
Ms Nash was disqualified by the court after it found on October 27 she held dual citizenship, which went against section 44 of the constitution.
A special count by the Australian Electoral Commission resulted in a win for Ms Hughes, who was the next candidate after Ms Nash on the NSW coalition Senate ticket for the 2016 election.
However, Ms Hughes was working as a part-time member of the Administrative Appeals Tribunal until October 27 when the court determined there was a vacancy.
Amicus curiae Geoffrey Kennett SC, in a written submission, argued the AAT job was an "office of profit under the Crown" - a reason for disqualification set out in the constitution.
Ms Hughes was incapable of being chosen because she held the AAT position from September 4-5, when the Senate resolved to refer the Nash case to the court, and October 27, he argues.
Solicitor-General Stephen Donaghue told the court on Wednesday the government accepted the AAT position was an office of profit under the Crown.
However, an "order of due election" by the court did not need to consider the issue of disqualification, he said.
The effect of the order was merely to allow Ms Hughes to take her seat, but did not preclude any future court case over her eligibility.
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".
As well, Ms Hughes was appointed to the AAT from July 1 this year, nearly 12 months after polling day.
The court heard Greens senator Andrew Bartlett had been duly elected via a court order which allowed him to replace Larissa Waters, who was disqualified because of her dual citizenship.
Senator Bartlett has fended off suggestions his university job precluded him from sitting in parliament under section 44 of the constitution.