The Greens have lost their second co-deputy leader in less than a week, with Queensland senator Larissa Waters announcing her resignation as a result of holding dual citizenship.
Waters said it was with "shock and sadness" she learned she was a dual citizen.
At a press conference in Brisbane, an emotional Senator Waters apologised to the Australian people saying she had discovered she remains a dual citizen of Canada and Australia. "As people would know after the recent departure of my dear friend and former colleague Scott Ludlam, section 44 of the Australian constitution means I can not hold office in the federal Parliament", Waters said in a statement.
Ms Waters, from the left-wing Greens party, was born in Canada.
Ms Waters' announcement follows the resignation of fellow WA-based Greens senator Scott Ludlam who failed to renounce his New Zealand citizenship.
However, Ms Waters, who was first elected in 2011, said she took "full responsibility".
In Australia, dual citizens are barred from election unless they can prove they took reasonable steps to sever foreign ties.
Her departure will potentially pave the way for the political return of former Australian Democrats leader Andrew Bartlett, who has since joined the Greens and was second on the party's Queensland senate ticket when Waters was re-elected past year. "I am deeply sorry for the impact that it will have", she said.More news: Neymar is uncomfortable at Barcelona
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Her parents had advised her she had until age 21 to seek Canadian citizenship, but never did so and thought the matter had ended there.
She had previously made headlines in 2016 as the first parent to breastfeed in the Senate Chamber.
"I apologise wholeheartedly to all those who have supported me and helped me to become a representative for the wonderful people of Queensland over the last six years", she said in a statement.
She said would continue to fight to protect the environment and fight for gender equality.
Her departure will trigger a Senate countback which could install the next candidate on the Greens ticket for Queensland, Andrew Bartlett, in the parliament.
"I won't sugar coat it, we need to make sure that our internal party processes are up to the challenge", he said.
Ms Waters' resignation is effective immediately.