Deputy Greens leader Larissa Waters has resigned from the Senate after realising she holds dual citizenship and should never have been elected to Federal Parliament.

Key points: Waters was born in Canada and did not renounce her dual citizenship

Waters was born in Canada and did not renounce her dual citizenship Waters says she believed she had been naturalised to Australia

Waters says she believed she had been naturalised to Australia Greens leader says he is "gutted" by Waters' resignation

Ms Waters was born in Canada and did not renounce her dual citizenship when running for office, making her ineligible to serve since her election in 2011.

The Queensland senator's resignation, effective immediately, comes less than a week after Scott Ludlam quit after realising he also held dual New Zealand citizenship.

Both avoidable resignations mean the Greens have lost two of their strongest performers, and the Senate now faces two recounts.

The constitution disqualifies potential candidates from election if they hold dual or plural citizenship.

Larissa Waters made history when she breastfed her baby in Parliament. ( AAP: Mick Tsikas )

The former Queensland senator was tearful when she apologised to her constituents for her oversight.

In a statement, Ms Waters said she had not visited Canada since leaving as a baby and believed she had naturalised to Australia.

She sought legal advice after Mr Ludlam's resignation and said she was shocked and saddened by the result.

"I was devastated to learn that 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.

"It is with a heavy heart that I am forced to resign as senator for Queensland and co-deputy leader of the Australian Greens, effective today.

Greens leader 'gutted' as both deputies resign

Sorry, this video has expired Richard Di Natale pays tribute to Larissa Waters

Party leader Richard Di Natale said he was "gutted" and said Ms Waters had "unfinished business".

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"This is unprecedented and deeply disappointing — personally for Larissa, for her family and for the Greens federally and in Queensland," he said.

Sorry, this video has expired Greens senator Scott Ludlam resigns over failure to renounce dual citizenship

"This is an innocent mistake, and Larissa has acted quickly and honestly to correct it."

Senator Di Natale pledged an "urgent root-and-branch review" of party processes to ensure no other parliamentarians were invalidly elected.

Ms Waters took full responsibility for the oversight and apologised for any embarrassments, saying it was no-one's fault but hers.

"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.

Ms Waters said she suspected other senators and MPs would need to resign, but was confident no-one else in the Greens would quit due to dual citizenship.

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Sorry, this video has expired Resignations raise questions over future Greens leadership, Insiders host Barrie Cassidy says

Ms Waters was seen by some in the party, including former leader Bob Brown, as a future leader of the party.

She was made co-deputy leader of the party in May 2015 and has been a strong opponent of the proposed Adani mine in the Galilee Basin.

She made international headlines earlier this year when she became the first woman to breastfeed her daughter, Alia, on the floor of Parliament.

It is not known whether Mr Ludlam and Ms Waters will be forced to repay their parliamentary salaries, likely to be well in excess of $1 million given the length of their tenure.

Former leader of the Australian Democrats Andrew Bartlett is in line to re-enter the Senate, as he would likely win a countback after Ms Waters' resignation.

Mr Bartlett, also the convenor of the Queensland Greens, said his "immediate focus" was to resolve the confusion Ms Waters' sudden departure had caused within the party.

"The party's membership will be having many conversations over the next few days as we process what has happened and determine what is the best way forward," he said.

Mr Brown said he would like Mr Ludlam and Ms Waters to return to Federal Parliament at some stage "down the line", saying they were very popular figures.

Greens need 'watchful eye': Brown

The former Greens leader also backed Senator Di Natale's review of the party's vetting processes, saying "from a very great crisis sometimes good things come".

"That root and branch structure review that Richard Di Natale is looking at will be backed by the Greens members and will lead to a much more professional and watchful eye being kept from a professional organisation that needs to be national and needs to be there for the Greens right across the country," he said.

"The Australian Greens' administration is largely voluntary.

"We don't have the watchful eye and national secretaries that the Labor Party and the Libs have, and we need to develop that.

"I've been keen on that for a long time and I believe that's something that will come out of this."