The Australian Greens will conduct an urgent review of party processes after co-deputy leader Larissa Waters joined Scott Ludlam and resigned over holding dual citizenship.
Senator Waters announced she was standing down after realising she held Canadian citizenship, just days after Senator Ludlam resigned after revealing he still held New Zealand citizenship.
"It's with great sadness that I have discovered that I am a dual citizen," she told reporters in Brisbane.
The outgoing Senator was born in Winnipeg, Canada, and has been in office since 2011.
Senator Waters said she sought legal advice immediately after Senator Ludlam's announcement, saying she was devastated by the realisation she was also affected by constitutional rules which forbid dual citizens from being elected to parliament.
She said she left Canada as a baby and had thought she had been naturalised as an Australian.
"Because of 70-year-old Canadian laws I had been a dual citizen from birth, and that Canadian law changed a week after I was born and required me to have actively renounced Canadian citizenship," she said.
"I had not renounced since I was unaware that I was a dual citizen. Obviously this is something that I should have sought advice on when I first nominated for the Senate in 2007, and I take full responsibility for this grave mistake and oversight.
Section 44 states any person who "is under any acknowledgement of 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 ... shall be incapable of being chosen or of sitting as a senator or a member of the House of Representatives."
Greens leader Richard Di Natale, who discovered Senator Waters' situation the same day as Senator Ludlam's resignation, said he was gutted and deeply saddened by the revelation and departure.
"This is unprecedented and deeply disappointing," he said.
"We had two individuals who made a mistake. It was a stuff-up. There is absolutely no other way of describing it. "
The Greens will conduct a root-and-branch review of its governance and internal processes, including how it vets candidates, to ensure this "never happens again", Senator Di Natale said.
Senator Waters has been co-deputy leader of the party since 2015.
Recently she made history by becoming the first woman to breastfeed in the Senate, and later, the first to breastfeed while moving a motion in the chamber.
She cited that as a highlight of her time in parliament, as well as preventing dredge spoil on the Great Barrier Reef, her anti-domestic violence advocacy and environmental issues.
"I have more to contribute and will be talking with my party about what lies ahead."
