The Australian Greens will urgently review its processes after losing two senators in less than a week over confusion about their citizenship.
Larissa Waters joined fellow deputy leader Scott Ludlam in announcing her resignation from federal parliament today.
The Queensland senator was born in Canada to Australian parents but left when she was 11 months old and has never returned.
She thought she had until the age of 21 to choose to be a Canadian, only to discover last week she had been a dual citizen from birth and had to actively renounce her citizenship.
That rules her ineligible to stand for election under section 44 of the constitution.
It's the same section that caught out Mr Ludlam, who stood down late last week after finding out he held dual citizenship with New Zealand.
Senator Waters assumed she was solely Australian when naturalised as a baby.
"I was devastated to discover that, in fact, the law had changed a week after I was born and I needed to have taken active steps to renounce what was my Canadian citizenship," she told reporters in Brisbane.
She admitted she should have sought advice when she first nominated for the Senate in 2007 and apologised for the "honest mistake" and oversight.
"It's the responsibility of candidates to check and I didn't check as closely as I should've," the Queenslander said.
"This is no-one else's fault but my own."