What law has Mr Ludlam fallen foul of?
Section 44 of the Constitution sets out restrictions on who can be a candidate for federal parliament. Five subsections make clear what disqualifies people, but it’s article one that tripped up Mr Ludlam.
It rules out anyone with “allegiance, obedience, or adherence to a foreign power, or is a subject or a citizen or entitled to the rights or privileges of a subject or citizen of a foreign power”.
“The general idea being that you need to give yourself wholeheartedly to your country and not have any mixed allegiance,” Professor Anne Twomey, a constitutional law expert at the University of Sydney, told SBS World News.
Technically, Mr Ludlam's New Zealand citizenship makes him a subject or citizen of a foreign power.
Was this situation avoidable?
Yes. If Mr Ludlam had renounced his New Zealand citizenship before he was nominated for pre-selection 10 years ago he could still be in parliament.
Mr Ludlam was born in New Zealand and settled in Australia aged nine. He was naturalised in his mid-teens and assumed that was the end of his New Zealand citizenship.
Crucially, however, he failed to take all reasonable steps to renounce his New Zealand citizenship - a precendent established by the Sykes versus Cleary case in 1992.
“Before nomination you have to take all the steps you can to renounce that foreign citizenship and that’s enough,” Professor Twomey said.
Have any other politicians been caught out by Section 44?
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 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 on Friday he was still required to renounce his “Kiwi status” before his political career.
Why does a multicultural nation like Australia still have this law?
Section 44 has been subject to review and debate in parliament, but nothing has come from it.
The main problem, according to Professor Twomey, is that a referendum is needed for any changes to the constitution.
“Nobody has thought that it was worth the money and effort to hold a referendum to make those changes,” she said.
What happens next?
The Senate is expected to refer Mr Ludlam’s election to the High Court, where it will likely be ruled invalid.
Professor Twomey said a recount of the 2016 WA senate vote will be required, rather than the Greens being able to choose a replacement.
University student Jordan Steele-John, the third person on the Green’s ballot, is touted as Mr Ludlam’s replacement.
