Labor senator Katy Gallagher has argued the delay in renouncing her British citizenship, caused by the UK Home Office demanding more documents, should not be grounds to have her kicked out of Parliament.

Key points: Lawyer says Senator Gallagher began process to renounce British citizenship in plenty of time

Lawyer says Senator Gallagher began process to renounce British citizenship in plenty of time Argues she took more steps to renounce than some of those previously excused

Argues she took more steps to renounce than some of those previously excused Case to be heard by the High Court in Canberra next week

Senator Gallagher's legal team have made written submissions to the High Court as she fights to hold on to her position in the Upper House amid the dual citizenship fiasco.

Last week, the Commonwealth argued the former ACT chief minister failed to do everything required to renounce her citizenship before nominations closed ahead of the 2016 election.

Former solicitor-general Justin Gleeson QC, who quit the role after a bitter public feud with then attorney-general George Brandis, is acting on Senator Gallagher's behalf.

He told the court Senator Gallagher began the process to get rid of her dual citizenship in plenty of time, ahead of nominations for the federal election closing in June 2016.

"As and from April 20, 2016, Senator Gallagher had made a declaration of renunciation in the prescribed manner required by British law and had provided 'information showing' each of the matters specified in British law," Mr Gleeson wrote.

"By May 6, 2016, all of her documents had been received, and the fee for her request was deducted by the Home Office."

Mr Gleeson said that in July 2016, the British Home Office demanded further evidence of Senator Gallagher's British citizenship, namely her father's original birth certificate.

Senator Gallagher's renunciation was finally registered in August 2016, more than a month after the election.

Lawyer says Gallagher went beyond 'reasonable steps'

The Commonwealth had also argued that the "reasonable steps" test, which demands all prospective politicians do everything reasonable to rid themselves of their dual citizenship, should only apply in circumstances where another country makes it impossible to renounce their nationality.

An example often provided is that of former Labor senator Sam Dastyari, who was unable to renounce his Iranian citizenship because the Iranian Government did not allow it.

But Mr Gleeson argued that claim, and likening Senator Gallagher's situation to that faced by the original Citizenship Seven politicians, was wrong.

"The various parties unsuccessfully invoking 'reasonable steps' in [the case of Nationals senator Matthew Canavan] did so in a context where each of them had taken no step required by the foreign law to terminate their foreign citizenship at the date the election commenced, i.e. nomination," Mr Gleeson argued.

"These parties invoked 'reasonable steps' to seek to excuse taking any steps, on the basis that they did not have sufficient knowledge that they needed to act.

"In contrast, Senator Gallagher is the first person this court has had to rule upon who has in fact taken steps required under the foreign law to renounce her citizenship at the date of nomination."

He argued that the Commonwealth's case put too much emphasis on the actions of an official of a foreign government in processing any renunciation claims.

"The Attorney-General's interpretation of the test wrongly diverts its focus away from what is required by foreign law and what is within the power of a citizen; towards what is not required by foreign law and what rests within the power of a foreign official," he wrote.

Senator Gallagher joined the Senate before the 2016 election, resigning as ACT chief minister to fill the position left by Kate Lundy.

Her opponents have argued her tenure in parliament before the last poll is further reason she did not act quickly enough to renounce her dual citizenship.

However, the High Court will only be deciding on her fate in this current term of Parliament.

Mr Gleeson represented former MP Tony Windsor in the original citizenship cases, as he successfully argued former deputy prime minister Barnaby Joyce should be ruled ineligible for Parliament because he was a dual citizen of New Zealand.

Some politicians are looking at Senator Gallagher's case as a potential precedent for other members and senators to have their eligibility judged by the High Court.

Among them is Labor MP Susan Lamb, who has been unable to renounce her dual citizenship because the UK Home Office is demanding she provide her parents' marriage certificate.

Ms Lamb claimed she was unable to produce that document because she is estranged from her mother, but her political opponents allege she could simply request a copy from the Queensland Registry of Births, Deaths and Marriages.

The High Court will hear Senator Gallagher's case in Canberra next week.