Critics have long argued the unusual restriction could pose problems for a multicultural country with one of the highest immigration rates in the Western world. Australia's population grew by 1.6 percent last year, mostly through immigration — the largest rise within the Organization for Economic Cooperation and Development group of 35 countries after New Zealand, which rose 2.1 percent.
The precaution against foreign interference in politics also raises questions about how fair it is that the citizenship policies of other countries could decide whether an Australian citizen can run for Parliament.
The clause in Australia's 116-year-old constitution that includes the ban "is a problem, it's broken in terms of its capacity to serve the needs of the modern community and it needs to be fixed," George Williams, Dean of Law at the University of New South Wales, said in an August speech.
"It's built for instability because of the society in which we now live," Williams said. He noted that three parliamentary committees in the 1980s and 1990s agreed that the ban should be lifted, and argued it should be removed from the constitution via a referendum.
The ban is unique in the Commonwealth of Nations, a group of mostly former British colonies that includes Australia. The United States, which along with Britain inspired Australia's hybrid parliamentary system, also welcomes dual citizens in Congress.
The ban too seems incongruous with Australia's lack of control over foreign influence through political donations that increasingly come from China. Last year, John Berry, then U.S. ambassador to Australia, said the United States was surprised by the amount of Chinese money and influence in Australian politics. An Australian parliamentary committee has since recommended a ban on political donations from foreign companies and individuals, but the government has yet to act with legislation.
Government lawyers have even suggested in submissions to the High Court that the dual citizen ban was a 19th century drafting error in the constitution. The court this week is exploring the ban in the case of seven lawmakers whose potential disqualification from Parliament could cost the government its slim legislative majority. The three-day hearing is due to finish Thursday.
The seven lawmakers include Deputy Prime Minister Barnaby Joyce and government ministers Matt Canavan and Fiona Nash. Four of the seven are Australian-born and inherited a second nationality from a parent. Three others immigrated as children.
All said they did not know they were dual citizens when they were elected last year. Canavan's Australian-born mother, with Italian parents, only became an Italian citizen due to a 1983 ruling by that country's Constitutional Court. Australia's High Court has heard conflicting evidence on whether Canavan had also become Italian by descent.
The deputy prime minister inherited his New Zealand citizenship from his father, but did not become aware of it until August, after journalists started asking questions about his background. If Joyce is disqualified from Parliament, it would force a by-election that could cost the ruling coalition its single-seat majority in the House of Representatives.
Joyce's lawyer Bret Walker said Tuesday that the dual national ban was designed to prevent "split allegiances" among lawmakers — a concern that did not apply to Joyce. "There's no split allegiance where you're not aware of one," he told the court. "You cannot heed a call you cannot hear."
But Brian Walters, a lawyer for the only two senators who have quit over their foreign citizenships, of Canada and New Zealand, told the court on Wednesday that the ban is designed to reinforce public confidence in Parliament.
"It's a provision that requires that the government of Australians will be by Australians and only by Australians," Walters said. The High Court has previously ruled that lawmakers who make "reasonable steps" to renounce foreign citizenship are exempt from the ban, acknowledging that some countries would always regard them as their own.
But the court has never before dealt with Australian-born lawmakers who did not ask for a second nationality. Government lawyers question whether the ban on someone who is "a subject or a citizen ... of a foreign power" was ever intended to disqualify such lawmakers.
The lawyers have produced an early draft of the constitution that would only have disqualified lawmakers for actively seeking citizenship of another country. The six British colonies that banded together to create the Australian federation in 1901 each had a parliament that allowed foreign citizens by descent. Those colonies are now Australian states that elect dual national lawmakers to their state legislatures.
Critics of the ban fear that a strict interpretation of it by the High Court could make Parliament less representative of the population by potentially creating more obstacles for migrants who want to become candidates.
An opposition senator spent $20,000 on lawyers to renounce his Iranian citizenship before he entered Parliament in 2013.