The High Court is yet to decide whether the Labor senator can remain in parliament.
Labor senator Katy Gallagher failed to renounce her UK citizenship before the last election because she left the application too late and did not attach the right documents, government lawyers have told the High Court.
Senator Gallagher's case was debated at a hearing on Wednesday before the full bench of judges, who will soon decide whether the senator should become the 10th politician to be kicked out of parliament in the ongoing dual-citizenship saga.
On Wednesday afternoon, the judges said they would "reserve their judgement".
The case centres on whether Senator Gallagher took "reasonable steps" to sever ties with the UK in the lead-up to the 2016 poll.
Her lawyers argue the ACT senator sent in "sufficient" documents to the UK Home Office - a copy of her Australian passport and birth certificate - before the writs were issued for the 2016 election.
"By no later than May 6, 2016... Senator Gallagher had taken every step which, as a matter of British law, was sufficient for her renunciation to be effective," the senator's lawyer Justin Gleeson said.
But on 20 July, weeks after the election, the UK Home Office asked for additional documentation to prove she was a British citizen through descent from her father.
Senator Gallagher then quickly sent in her father's birth certificate, her parents' marriage certificate and her own birth certificate.
Solicitor General Stephen Donaghue, representing the government, said the senator only submitted her application 41 days before she nominated and should have expected the process to take time, especially given she was pre-selected as a Labor candidate a year before the election.
"Because the application was not submitted a sufficient time in advance to be perfected under British law... reasonable steps were not taken," Mr Donoghue told the court.
Mr Donoghue also drew the judges attention to a UK government document that specifically requests citizens by "descent" from a parent or grandparent, like Senator Gallagher, to "send documents proving that person's citizenship or status and your relationship to him or her".
In response, the Labor senator's lawyers said the ACT birth certificate sent with the original renunciation form listed her father's UK birthplace, and therefore was sufficient proof he was a UK citizen.
Mr Donaghue argues the previous dual-citizenship cases had established a high standard for senators and MPs, where they should be expected to meet the foreign government's standards to fully renounce citizenship unless there is an "irremediable incapacity" to comply.
Last year, the High Court pardoned Coalition senator Matt Canavan and independent senator Nick Xenophon who both argued they were victims of complex foreign citizenship laws.
The Gallagher case could have implications for other MPs who were still dual citizens but seeking renunciation when they nominated.
The government wants the senator's fellow Labor MPs Justine Keay, Josh Wilson and Susan Lamb to have their citizenship matters referred to the High Court.
There are also calls for crossbencher Rebekha Sharkie and Liberals Julia Banks, Jason Falinski, Alex Hawke and Nola Marino to be referred.