Canadian-born deputy Greens leader Larissa Waters is resigning from the Senate after realising she had not renounced her dual citizenship when running for office.
Here's a look at her political career up to now.
When was she elected?
She was elected to the Senate for Queensland in 2010, and started her first term on 1 July, 2011. She was re-elected in 2016.
In her maiden speech to Parliament in 2011 she said "the Greens were the only choice".
"No other political party captures my beliefs and values so entirely, operates with unfailing integrity and honesty, stands up for what is right even if it is controversial, and has living within our ecological means and treating each other with more kindness as its central tenets."
She went on to say, "When my time is up, I leave this place having contributed in some small way to improving our environmental laws with better community rights, consideration of cumulative impacts, and federal oversight of water, the lawyer in me will be delighted."
Before entering politics she was a legal researcher for the Land and Resources Tribunal and a community environmental lawyer in the Environmental Defender's Office between 2002 to 2011.
What has her role been with the Australian Greens?
She was made co-deputy leader of the Australian Greens in May 2015.
She's been a spokesperson on a range of different issues, most recently in environment and biodiversity, mining and resources, women, gambling and tourism.
She's also been a key voice in issues including coal seam gas, the Great Barrier Reef and climate change.
What's she best known for?
In May, Larissa Waters made headlines around the world when her daughter Alia became the first baby to be fed on the floor of Federal Parliament.
"I am so proud that my daughter Alia is the first baby to be breastfed in the Federal Parliament," Senator Waters said at the time after parliamentary rules were changed in 2016 to allow mothers to feed their children in the chamber.
She continued to make history a month later, when she put forward a motion on black lung disease in the Senate while breastfeeding, and was praised widely on Twitter for paving the way for more women to breastfeed at work.
"First time I've had to move a Senate motion while breastfeeding!" she tweeted afterwards.
"And my partner in crime moved her own motion just before mine, bless her."
What did she say today?
In a statement, Ms Waters said she discovered the error "after Scott's shock discovery" and sought legal advice.
She was born in Winnipeg, Canada, in 1977, but left as a baby.
"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," Ms Waters said.
"I have been incredibly fortunate to be able to represent my values and speak for Queenslanders who want a fairer and cleaner world.
"There is no greater honour than to be entrusted with that responsibility and I have discharged it to the best of my ability, with the support of many."
What has Greens Leader Richard Di Natale said?
He tweeted that he was "gutted" before fronting the media this afternoon.
"I just want to say we don't want to make excuses for what happened," he said.
"That is something that I think in time people will look at, the circumstances of surrounding Larissa are clear. Born to Australian parents, somebody who was 11 months old when she came to the country.
"We all know the rules as they stand and we had two individuals who made an oversight and are paying for it.
"They are paying a high price for it."
Does this mean her political career is over?
That's an unanswered question.
In a press conference this afternoon Ms Waters said she would consult with her party.
ABC political reporter Jane Norman told News 24 it wasn't the first time a politician had been re-elected in similar circumstances.
"For Larissa Waters, she can renounce the citizenship which she said she would do today and run again," Ms Norman said.
"It is not the first time this has happened.
"Jackie Kelly, a Liberal member many years ago did that and ran for parliament again and was elected."
And it seems Senator Di Natale would be only too happy to have her back.
"As far as I am concerned, I would welcome her back to the Parliament with open arms," he told a press conference this afternoon.
"She is an incredible person.
"She is somebody who has represented the people of Queensland with such passion."