A second Australian senator in less than a week has said she is quitting parliament after discovering she was a dual national and had therefore never really been elected.
The controversy has raised questions about how many other politicians might also have no right to be there.
Larissa Waters, co-deputy leader of the Greens party, said she was quitting after six years as a senator after the Canadian High Commission in Canberra told her on Monday that she was Canadian.
On Friday, the Greens' other co-deputy, Scott Ludlam, revealed that he was a citizen of New Zealand as well as Australia, which made him ineligible for the Senate job he has held since July 2008.
Australia's constitution states that a "citizen of a foreign power" is not eligible to be elected to parliament.
Ms Waters, who in May became the first politician to breastfeed in parliament, was born in the Canadian city of Winnipeg on February 8 1977 to Australian parents.
She moved to Australia before her first birthday.
Ms Waters said she thought she had an option of becoming a Canadian citizen and did not take it.
She has since found that the law changed a week after she was born, meaning she automatically became a Canadian unless she took steps to prevent it.
Ms Waters said other foreign-born politicians among the 226 in parliament could find themselves in a similar predicament.
"There are many politicians in the Senate and the federal House of Representatives that were born overseas and it may well be that others have to make this embarrassing revelation as well," an emotional Ms Waters told reporters.
"But I can hold my head up high knowing that the moment I found out, I have taken the step of announcing that I will sadly have to resign," she added.
After Mr Ludlam's resignation, government politician and former prime minister Tony Abbott posted on social media a document confirming he had renounced his own British citizenship in 1993, a year before he was elected to parliament.
Sydney University constitutional lawyer Anne Twomey said Canada and New Zealand were not considered foreign powers when the Australian constitution came into force in 1901 because, like Australia, they were part of the British Empire.
The High Court has ruled that Britain has been regarded as a foreign power under Australian law since 1986.
One or two prime ministers might have been dual nationals in the past, but politicians' eligibility was never tested in the courts until the 1970s, Ms Twomey said.
"Up until then, people didn't look too closely. It's only been in recent times when perhaps politics has become a bit more visceral ... that these issues have arisen," Ms Twomey told the Australian Broadcasting Corporation.
Mr Ludlam was exposed by Australian lawyer John Cameron, who had investigated whether the senator and another New Zealand-born politician, Derryn Hinch, were dual nationals.
Mr Hinch, an independent senator, had renounced his citizenship.
The Greens are likely to retain all eight Senate seats and their balance-of-power role despite the two resignations.
A recount of votes at the last election a year ago will probably deliver the seats to other Greens candidates.
But both Mr Ludlam and Ms Waters could be forced to repay their salaries.
Mr Ludlam was reportedly paid more than 1.6 million Australian dollars (£966,000) during his nine years in office.
Greens leader Richard Di Natale said that he was "gutted" by the loss of his deputies and that the party will tighten procedures to prevent ineligible candidates from running again.