Fresh doubts have been cast over Labor's claims about the strength of its citizenship verification procedures after legal advice from a leading citizenship expert found an MP was a dual citizen when she was first elected in 2015.
According to former British home office lawyer Phillip Gamble, Labor senator Katy Gallagher, who earlier this year acknowledged she was eligible for British citizenship, would have been a British citizen when she first took her seat in Parliament.
The Labor Party has insisted its protocols are beyond reproach as Opposition Leader Bill Shorten and Prime Minister Malcolm Turnbull prepare to discuss the details of the proposed citizenship register in Melbourne on Tuesday.
Senator Gallagher renounced her right to British citizenship prior to the 2016 election in what she described as "an abundance of caution" but Mr Gamble, a UK citizenship expert, says she was already a British citizen when selected to fill a casual vacancy in 2015 as a result of acquiring it automatically from her British-born father.
"If the senator's father was born in the UK then she would have acquired British nationality by operation of law at her birth," Mr Gamble told Fairfax Media, applying the same argument that saw senators Stephen Parry and Fiona Nash disqualified from office.
Senator Gallagher has also faced persistent questions over the possibility she acquired Ecuadorian citizenship through her Ecuadorian-born mother, whose birth was registered in the British consulate in Ecuador in 1943.
The former ACT chief minister said her legal advice was her mother was never eligible for Ecuadorian citizenship despite the 1929 and 2008 constitutions granting those born in Ecuador nationality by birth, regardless of their lineage.
There is no threat of Senator Gallagher being referred to the High Court over her British citizenship as it can only consider eligibility in the current Parliament but Mr Gamble's opinion raises further questions over the robustness of Labor's processes.
Senator Gallagher has also refused to release her declaration of renunciation certificate raising questions whether her application had been fully processed when the 2016 election was called six weeks later.
"I was advised that submitting the declaration of renunciation to the Home Office meant that I had taken all reasonable steps to renounce any entitlement to British citizenship," she told the Senate in September.
Two other Labor MPs in the House of Representatives born to British fathers, Justine Keay and Susan Lamb, also remain under a possible citizenship cloud after refusing to release the date their renunciation was registered by the British Home Office.
Constitutional law experts, George Williams and Anne Twomey have told Fairfax Media that if the two MPs' applications to renounce were processed after June 7, when nominations for the 2016 election closed, the pair may not have been validly elected under a strict interpretation of the constitution by the High Court.
"The temporal focus for the purposes of s 44(i) [which pertains to dual citizens] is upon the date of nomination [June 7, 2016] as the date on and after which s 44(i) applies until the completion of the electoral process," the High Court found in October.
Ms Keay revealed on radio in August that she did not receive confirmation she had renounced her citizenship until July 11, a week after the federal election.
Prime Minister Malcolm Turnbull said if Ms Keay did not want to resign it was "going to involve some considerable legal debate".
"I think the High Court's been very clear on the matter in the recent decision. It is a very tough, very literal decision that's been taken," he said on Wednesday.
Mr Shorten said "he was not about to take legal advice from Mr Turnbull".
His office refused to release the documentation of Ms Lamb or Ms Keay, stating: "Labor has strong processes and we welcome greater transparency and disclosure."
Labor is the only major party yet to have a federal member referred to the High Court.