SYDNEY • Australian Senator Nick Xenophon, a key independent lawmaker, is resigning from national politics, potentially complicating Prime Minister Malcolm Turnbull's efforts to pass legislation.

Mr Turnbull does not have a majority in the Senate and has relied on Mr Xenophon's party, the fourth-largest bloc in the Upper House, to pass legislation.

Mr Xenophon's replacement will come from his party, which bears his name, but is likely to be far less experienced compared with his nine years as a senator.

"Xenophon was a known quantity, he was experienced and... Turnbull could work with him," Dr Peter Chen, senior lecturer in politics at the University of Sydney, was quoted as saying by Reuters.

"His potential replacement will be inexperienced, and that can be unpredictable. This will not be Turnbull's preferred choice."

Mr Xenophon's future as a senator was under a cloud due to him holding possible dual citizenship. Under the Constitution a lawmaker must hold only Australian citizenship to be eligible to be elected to the national Parliament.

Mr Xenophon had renounced his Greek citizenship, which he received from his mother, but had not been aware he held possible British citizenship. While he was born in Australia, his father was born in Cyprus, which was a British colony until independence in 1960.

Mr Xenophon's citizenship issue was due to be determined by Australia's High Court. The court will next week rule on the eligibility of seven politicians, including Mr Xenophon and Deputy Prime Minister Barnaby Joyce. Should Mr Joyce be ruled ineligible, Mr Turnbull will lose his Lower House majority.

In announcing his surprise resignation, Mr Xenophon said he would stand for election in his home state of South Australia next year. State Parliaments do not have the same citizenship rule as the national Parliament.

"I have increasingly concluded that you can't fix South Australia's problems in Canberra without first fixing our political system at home. South Australia politics is broken and politically bankrupt," he told reporters in Adelaide.

The timing of his announcement, however, inevitably fuelled suspicions that Mr Xenophon is pessimistic about his chances in court and moved to avoid the embarrassment of a negative result ahead of the hearing, according to the Sydney Morning Herald.

But he has insisted that he expects to win his case in the High Court and instead said that he had actually delayed his Senate resignation as a result of the case.

He also described the decision as a big gamble, saying he was in a fight for his political life.