Greens senator Scott Ludlam is leaving federal parliament after finding out he was improperly elected more than a decade ago.
Ludam said he apologised "unreservedly" for his mistake and was devastated by the consequences.
The deputy leader of an Australian political party has announced he is ending his nine-year career in Parliament because he has discovered he has technically never been a senator.
Prime Minister Malcolm Turnbull has described Scott Ludlam's oversight regarding his New Zealand citizenship as "remarkable".
The highly regarded barrister obtained proof Senator Ludlam still holds New Zealand citizenship and is ineligible for politics in Australia.
Ludlam, born in Palmerston North, moved to Western Australia when he was 3, and only discovered he was a dual citizen last week.
Mr Turnbull was asked if Mr Ludlam would be required to pay back his salary.
"It's history making, there's three senators have now been ruled out all by different sections, different parts of section 44 of the constitution - we've had a disqualification for a conviction, one for a conflict of interest, and now one for citizenship".
"This was my error, something I should have checked when I first nominated for preselection in 2006", he said in a statement published to his Twitter account.
"It's going to be millions of dollars and my total assets amount to a fast computer and some nice shoes", he told reporters in Perth on Friday.
"I was naturalised [to Australia] in my mid-teens".
It then took Senator Ludlam's office several days to contact the High Commission in New Zealand and verify the information was correct.
He added, "It wasn't the way I was hoping to go out". I'll really miss it, but there are other ways to make trouble.
Greens leader Richard Di Natale said he was "devastated" by Senator Ludlam's discovery.
As a colleague, Scott has been an outstanding member of the Parliament and of the Greens.
"He has been a strong representative for the people of WA and the nation on a range of issues".
'He will continue to be a champion of the Greens movement and a dear friend'.
Mr Ludlam's position will be filed by a recount of ballot papers from the 2016 election, leading to the possible election of 22-year-old Jordan Steel-John who was third on the Greens Senate ticket.
Mr Steele-John, who has cerebral palsy, had to give up his British citizenship to run for parliament back in 2013.
Under the constitution, a dual national can not stand for election.
On Friday, Mr Ludlam quit after accepting he had been ineligible to be a senator since his election in 2008.
Fellow Western Australian and former One Nation senator Rod Culleton was booted from the Senate earlier this year after a Federal Court ruled his business has gone bankrupt.