Labor MP David Feeney has been caught out by the dual citizenship fiasco, admitting he has no documents to prove he renounced his British citizenship and pledging to refer himself to the High Court unless he can produce them this week.
The revelation raises the prospect of a byelection early next year in his Melbourne seat of Batman, which could easily fall to the Greens. It is also an embarrassment for the ALP, which has long claimed its processes were watertight.
The development came as the Turnbull government ramped up pressure on Opposition Leader Bill Shorten to also refer Labor senator Katy Gallagher to the court.
A handful of other politicians on both sides are under renewed suspicion after the citizenship documents of all House of Representatives MPs were published late on Tuesday. As many as five others could also face the High Court, triggering a possible 'super-Saturday' round of byelections in early 2018.
But Mr Feeney's situation was the most obviously problematic, and he immediately stood in federal Parliament to admit he was in trouble.
"I accept I have been unable to produce the requisite notice of renunciation with respect to the United Kingdom," he said. "I remain hopeful that continuing searches of UK records and archives will clarify this matter in my favour.
"Nonetheless I accept that at this moment my status as a citizen under UK law remains unclear."
Mr Feeney said if documents were not located soon, he would ask Parliament to refer him to the High Court for a final verdict.
Finance Minister Mathias Cormann said Opposition Leader Bill Shorten has been exposed as a "sanctimonious hypocrite" for repeatedly claiming Labor MPs had no citizenship issues.
"You've got to assume Bill Shorten has known for some time David Feeney had this problem. It just completely exposes the hypocrisy on this issue," he said.
Labor legal affairs spokesman Mark Dreyfus accused the Turnbull government of conducting a cover up and claimed as many seven Coalition MPs faced questions over their citizenship status.
"We have a continuing concealment, a continuing cover up from Malcolm Turnbull on behalf of the seven MPs," he said.
He revealed Labor was even prepared to refer to court Environment and Energy Minister Josh Frydenberg, whose parents arrived in Australia stateless after escaping the Nazis.
But Mr Dreyfus could not say when Mr Shorten first learned Mr Feeney faced questions over his citizenship status. Mr Dreyfus also wrongly claimed the opposition leader had never guaranteed no Labor MPs would be caught up in the citizenship saga.
Mr Feeney has a history of sloppy paperwork. Fairfax Media revealed during last year's federal election campaign that he owned an undeclared $2.31 million property, potentially placing him in "serious contempt" of the Parliament.
Mr Feeney said prior to nominating as a senator in October 2007 he sought legal advice from the ALP about possible British or Irish citizenship. He was advised by the ALP legal unit that he was likely to be a British citizen by descent through his father.
He said he signed renunciation documents and "as far as I am aware" they were sent to the relevant authorities in Britain and Ireland.
But he never got any confirmation that his renunciation was successful and only checked again last month - six months into the dual citizenship crisis.
He said he is satisfied he is not an Irish citizen and took "reasonable steps to renounce any British citizenship". However the reasonable steps argument may not cut muster with a black-letter ruling from the High Court.
The fate of a number of MPs could be decided by what the High Court considers to be "all reasonable steps" to renounce citizenship of another country.
Tasmanian Labor MP Justine Keay confirmed her renunciation of citizenship had not been registered with the Home Office until a month after nominations closed for the 2016 election, potentially ruling her ineligible to sit in the current Parliament.
Fellow Labor MPs Josh Wilson and Anne Aly also received confirmation of their renunciations after the nomination date.
For Labor MP Susan Lamb, it is unclear if she was ever even eligible to take up UK citizenship. In a letter from the British Home Office on August 10, Ms Lamb was advised Britiain could not be "satisfied from the documents available that you hold British citizenship."
Two MPs with Polish heritage - Labor's Emma Husar and Liberal Jason Falinski - have requested their entitlement to Polish citizenship be renounced, but have not provided any response from Polish authorities confirming this had occurred.
Mr Falinksi told Fairfax Media: "My father was never Polish. You might as well ask me why there's no confirmation from the Ukranian embassy. As far as the UK goes, feel free to ask them yourselves. I'm literally not hiding anything."
At least four other MPs have sought legal advice over their citizenship status with Alex Hawke, Nola Marino, Julia Banks and Anne Sudmalis all requesting further clarification.
Mr Hawke and Ms Banks received advice that they had to be registered with the Greek municipal government in order to be considered a citizen despite being eligible.
Coalition MP Ann Sudmalis provided legal advice that as she had been born to a British mother, she was not eligible for automatic British citizenship - unlike those born to British father such as former Bennelong MP John Alexander.
Chief government whip Nola Marino - who Fairfax Media revealed last month may have acquired Italian citizenship through her husband - has sought "legal advice to confirm her citizenship status through marriage."
"I remain assured that I am not an Italian citizen through marriage," she wrote in her declaration form. She has not provided conclusive documentation despite claiming her husband lost his Italian citizenship when he naturalised as an Australian in 1958.
Referrals to the High Court would likely take place on Thursday or Friday, after the Parliament has passed same-sex marriage legislation.