The High Court has found Liberal Hollie Hughes is not eligible to replace the Nationals' Fiona Nash in the Senate.

Ms Hughes was identified as the replacement from a recount after the High Court's citizenship ruling on the former deputy Nationals leader.

But questions were raised over whether she was eligible because she took a government job on the Administrative Appeals Tribunal after losing the election.

The court has heard Ms Hughes resigned from the tribunal 45 minutes after Ms Nash's disqualification, in an attempt to ensure she would be an eligible candidate.

On Wednesday, lawyers for the Commonwealth argued she should be eligible to sit in the Senate because the process of election ended on polling day.

Earlier, Ms Hughes's lawyer, Arthur Moses, said: "Once the votes are cast a choice is made."

"No matter which way one looks at it Ms Hughes was eligible."

Solicitor-General Stephen Donaghue said Ms Hughes was a valid candidate, because she was not in breach of the constitution at the time of the election or the recount.

"Our submission is … that the process of choice ends with the polls," he said.

But the court found that is not the case, dismissing the summons to have her declared a senator, with reasons to be delivered later.

Lawyers employed to advise the court said the election was ongoing and Ms Hughes held a government job at a relevant time.

NSW Liberals 'examining court's decision'

Lawyer Geoffrey Kennett rejected suggestions from Justice Virginia Bell that it was odd to reject a candidate who had been eligible at the time of the poll.

"There's a risk to responsible government," Mr Kennett said.

"It may be harsh, but there are matters of constitutional principles involved."

Mr Donaghue questioned whether the court should be delving into the details of the case.

He acknowledged there was a discretion for the court to rule on the issue but warned of the consequences.

"There's good reason for the court to keep a tight rein on the circumstances," he said.

He told the court it had moved with astonishing speed to resolve the case but that would not always be the case, and there could be delays in filling seats made vacant while the court ruled on eligibility.

In a statement, the NSW branch of the Liberal Party said it was "closely examining the court's decision".

"We commend Hollie Hughes for the proactive and transparent approach she had taken, seeking the clarification of her status from the High Court," the statement said.

"The Party will now await further direction from the High Court."