The Greens Senator Scott Ludlam has not lived in New Zealand since he was three, but the fact that he continued to hold dual citizenship has ended his near decade-long political career.
The party's co-deputy leader was in New York last week addressing a UN conference on banning nuclear weapons, when his office was notified about the dual citizenship.
Under section 44 of the constitution, that makes him ineligible to hold elected office.
"Someone had done some digging for whatever reason," Senator Ludlam said, adding that he understood the person was an interested community member.
"I apologise unreservedly for this mistake."
He has chosen to resign rather than try to fight what he said was a ridiculous oversight and his fault.
He said that when he became an Australian citizen in his mid-teens he assumed he had given up his New Zealand citizenship.
"I could have dug my heels in I guess but it creates a messy, protracted dispute when that section of the constitution is crystal clear and has been tested before ... it's not something I particularly want to put myself or my staff through," he told reporters.
A nine-year senator never being eligible in the first place is believed to be unprecedented and the Department of Finance could pursue him over taxpayer salaries and allowances provided over that time.
However such debt was waived for Rod Culleton and John Day, who were both disqualified as senators this year.
Senator Ludlam took a couple of months off at the end of 2016 to deal with depression and anxiety.
However he was philosophical and positive on Friday, describing the decision as a far simpler situation than last year's.
"It is still settling in a little bit, it has been an incredible run and I have enjoyed every minute of it and I think probably when it all goes quiet or when they ring the bells on my colleagues in August and I'm not there it is going to settle in," he said.
He said he "would find some other way of stirring up trouble" but it was too soon to contemplate a comeback.
His biggest highlight was getting a nuclear waste dump "off the shoulders of old Aboriginal women in the NT".
"In my inaugural speech in September 2008 I said I wanted to be a part of a movement that turned the ship and I know it can be hard to tell at times but I feel like we are a part of doing that and we have done that," he said.
Greens leader Richard Di Natale said he was devastated at the news, but Senator Ludlam's decision to deal with the issue directly and immediately showed his integrity and character.
"Scott has been an outstanding member of the parliament and of the Greens," he said in a statement.
University student Jordon Steele-John, who has cerebral palsy, has been touted as the frontrunner to take the vacant seat.
"Like everyone else in the party I'm going to be spending the next week in sad shock and/or swearing loudly into a pillow," the 22-year-old posted on Facebook.
