It started with the shock resignation of WA Greens senator Scott Ludlam in July.
A surprise New Zealand citizenship - a legacy from his childhood - had rendered him invalidly elected under Section 44 of the Constitution.
In the weeks that followed, foreign-born and second-generation MPs and senators from across the political spectrum poured through their records, checking whether they too were dual citizens.
It turns out the Greens were not alone.
Pollies head to the High Court
Five politicians have already been referred (by their own parties) to the High Court, where their political fates will be determined in a hearing from October 10-12.
They include the deputy prime minister Barnaby Joyce.
His case is crucial, not just because of his high profile, but because he sits in the House of Representatives where the Coalition has a slim one-seat majority.
If the court finds Mr Joyce’s election was invalid, it would trigger a by-election in the seat of New England, which could threaten that majority and prompt a full election.
The other four are senators: Matt Canavan of the Nationals, Malcolm Roberts of One Nation, and the two ex-Greens senators Larissa Waters and Scott Ludlam - both of whom resigned.
When the Senate resumes in early September, two more senators will be referred: key crossbencher Nick Xenophon, who leads a party in his name, and the deputy Nationals leader Fiona Nash.
What’s wrong with being a dual citizen?
Nothing – unless you are a federal politician.
Section 44(i) of the Constitution disqualifies a person from sitting in the Australian parliament if they are a “subject or a citizen of a foreign power”.
It also applies if a person is “entitled to the rights or privileges” of a citizen.
Many of the politicians referred to the High Court are citizens by descent – they were not born in the country, but inherited the rights of citizenship through their parents.
What will the High Court consider?
It depends on what the lawyers argue, but we do have some clues.
A legal precedent from 'Sykes v Cleary 1992' holds that Section 44(i) does not apply if you take “all reasonable steps” to renounce your citizenship.
The High Court will now need to determine what was reasonable in each case.
Many of the politicians claim they did not know they were dual citizens in the first place.
Solicitor General Stephen Donaghue QC, who represents the government, told a directions hearing he would argue Mr Joyce, Senator Canavan and ex-senator Waters should not be disqualified.
He said those three cases were similar because none were aware they held foreign citizens.
Matt Canavan’s lawyer said he would argue citizenship by descent should not count. He said it would be “ridiculous” to exclude so many Australians from sitting in parliament.
Why did some resign while others did not?
The Greens party has maintained its two senators did the right thing by resigning when they discovered they were dual citizens.
Matt Canavan, the Nationals senator, stepped down from his cabinet position as resources minister – but he decided to remain in the Senate until the High Court decides his case.
He claims his mother signed him up for Italian citizenship without his knowledge or consent.
Barnaby Joyce is standing firm. He will keep his cabinet position and remain in the House of Representatives until the court decides.
Labor has pressured the government to explain the double-standard. The government said it was acting on legal advice.
Malcolm Roberts, Fiona Nash and Nick Xenophon then followed their example, choosing to remain in their positions until the court reaches a decision.
Have politicians been caught out by Section 44 before?
One Nation candidate Heather Hill was elected to the Senate in 1998, but the High Court later ruled her ineligible on the grounds she held dual citizenship of Australia and the United Kingdom.
Robert Wood was elected in 1987 to take his seat in the NSW Senate in 1988. However, his election was ruled invalid by the High Court because Mr Wood was not an Australian citizen at the time of his election.
Recently, One Nation Senator Rod Culleton was caught out by another article under Section 44. He was disqualified from the senate for being declared bankrupt - something prohibited under article three.
Have other Australian politicians been forced to renounce their citizenship?
Former prime minister Tony Abbott, who was born in London, renounced his British citizenship before becoming an MP in 1994.
Another former prime minister, Julia Gillard, who was born in Wales, renounced her citizenship before entering parliament in 1998.
Senator Derryn Hinch left New Zealand in 1963 and became an Australian citizen in 1980. He tweeted he was still required to renounce his “Kiwi status” before beginning his political career.