Greens senator Scott Ludlam is leaving federal parliament after finding out he was improperly elected more than a decade ago.

The party's co-deputy leader said it was recently brought to his attention that he holds dual citizenship of Australia and New Zealand, the Daily Mail reported.

Under section 44 of the constitution, that makes him ineligible to hold elected office.

"This was my error, something I should have checked when I first nominated for preselection in 2006."

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Instead of going through protracted legal proceedings, he is resigning as a senator for Western Australia and co-deputy leader of the Australian Greens.

Senator Ludlam was born in Palmerston North and left the country with his family when he was three.

He settled in Australia not long before his ninth birthday, before being naturalised when he was in his mid-teens.

"(I) assumed that was the end of my New Zealand citizenship.

"It is entirely my responsibility - it wasn't the way I was hoping to go out."

He lauded Senator Ludlam's intelligence on issues from digital rights to homelessness.

Scott Ludlam's letter.
Scott Ludlam's letter.

"He will continue to be a champion of the Greens movement and a dear friend."

The Senate is expected to refer the matter to the Court of Disputed Returns, which the Greens think will call for a countback of votes from the 2016 election.

University student Jordan Steele-John, 21, has been touted the frontrunner to take the vacant seat.

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Mr Steele-John, who has cerebral palsy, had to give up his British citizenship to run for parliament back in 2013.

Senator Ludlam is the fourth senator to leave the upper house this parliamentary term.

Bob Day and Rod Culleton were both ruled ineligible for constitutional reasons, while Liberal senator Chris Back resigned.