Senator Gallagher was not in court on Wednesday when government lawyers urged the full bench to find her ineligible as a consequence of leaving it too late to renounce her British citizenship before the 2016 election.

Solicitor-General Stephen Donaghue said it was clear Senator Gallagher had not complied with renunciation requirements and as a result her Senate seat should be vacated.

But her legal team says she took every action she could to sever citizenship by descent through her father and it was "incredibly uncertain" how long a person should have to wait for British officials to approve the decision.

Senator Gallagher submitted her renunciation application on April 20 and was charged a fee for it on May 6 - 10 days before federal election writs were issued.

Her lawyers argue she provided sufficient details but the British Home Office requested further information on July 20, almost three weeks after the election.

Dr Donaghue said renunciation processing times ranged between 55 and 60 days - and in some cases, up to six months - but Senator Gallagher left just 41 days for hers.

"On those facts alone, your honours could conclude she did not take all reasonable steps," he told the court on Wednesday.

Senator Gallagher was preselected 12 months before the election and 11 months before she nominated.

"If a candidate chooses to leave it too late ... they'll suffer the consequences," he said.

Dr Donaghue also argued the documents Senator Gallagher initially provided - copies of her passport and birth certificate - were insufficient and it had been clear she must provide proof of citizenship through copies of her father's birth certificate and parents' marriage certificate.

That point is vehemently opposed by the senator's legal team.

Justin Gleeson SC said she was entitled to believe her birth certificate was enough to prove her father was a British citizen.

He also argued the test in section 44, which deals with dual citizenship, requires a person to take the reasonable steps available to them to renounce.

She had and the decision rested with British authorities, he said.

"It can take days, months, years ... you can try to hurry that up but you can't control it," he said.

He suggested finding Senator Gallagher ineligible would mean people must start the renunciation process so far in advance of an election they might not have made a firm decision to stand.

Dr Donaghue earlier warned if "reasonable steps" was a sufficient test, people could game the system and drop the renunciation if they are not elected.

Australian Associated Press