Australians in a suburb of Sydney are voting in a by-election that could cost the governing conservative coalition its slim majority in parliament.
The poll was called after Bennelong MP John Alexander stepped down in the crisis over dual citizenship.
The citizenship crisis has forced nine Australian MPs to resign.
Mr Alexander, of the Liberal-National coalition, is standing for re-election but faces a strong challenge from Labor’s Kristina Keneally.
If Mr Alexander loses, Prime Minister Malcolm Turnbull would lose his one-seat majority in parliament.
The government would then have to rely on the support of five independents to push through its programme.
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“People will be casting a judgment on the government, which I lead, of course,” Mr Turnbull told reporters.
Australia’s constitution demands politicians are not dual nationals.
A key factor in the vote could be how Chinese-born voters decide. They make up more than a fifth of the electorate in the constituency.
The opposition has accused the government of “China-phobia” after Mr Turnbull accused China of interference in Australia.
Turnbull’s citizenship plans
Mr Alexander, a former tennis star, resigned after revealing that he had probably inherited dual British citizenship through a UK-born parent.
He said at the time that he could “no longer… maintain the belief” he only held Australian citizenship. However, it emerged that he might not even be entitled to UK nationality and so he is re-contesting the seat.
Among the politicians unseated by the citizenship crisis was Deputy Prime Minister Barnaby Joyce. Mr Joyce easily won a by-election earlier this month and returned to parliament.
Mr Turnbull has unveiled plans to make all federal politicians clearly declare their citizenship status to avoid a similar crisis in the future.
Under the new plan, politicians will be obliged to make a formal declaration about their citizenship status, as well as provide details about the time and place of their birth, and the time and place of birth of their parents.
If any politicians formerly had citizenship of another country they will also be required to detail when and how they renounced it.