Former Greens senator Larissa Waters, who was forced out for being a dual citizen, is set to return to Federal Parliament.
Her replacement, Andrew Bartlett, has announced he will quit the Senate at the end of August to focus on campaigning for the Lower House seat of Brisbane.
One of the Greens' strongest performers, Ms Waters spent six years in Canberra before resigning last year when realising she was a dual citizen, in breach of Section 44 of the constitution, and should never have been elected.
In a statement, Ms Waters said she was thrilled to, "become the first senator section 44'd to return to the seat she won".
"I've had a year watching politics from the outside and it's clear that people are fed up with the big parties just doing the bidding of their corporate masters and donors," Ms Waters said.
"I want to thank my old friend and esteemed colleague, Senator Andrew Bartlett, for the outstanding work he has done in the Senate representing Queenslanders since he stepped into the role, after I stood down for unwitting dual citizenship."
Senator Bartlett said he was quitting to focus on campaigning for the Lower House seat of Brisbane.
"The strong swings and successes the Greens achieved in the recent state election in Queensland shows we have a real chance of breaking through in a Lower House seat in the next federal election, but it will take a concerted community-based campaign to do it," Mr Bartlett said in a statement.
Ms Waters has been seen by some in the party, including former leader Bob Brown, as a future leader of the Greens.
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.
Senator Bartlett said he would work with Ms Waters in the next two months "to ensure the transition occurs with minimal disruption".
Along with the resignation of her former colleague Scott Ludlam, Ms Waters' resignation helped spark the constitutional crisis over federal politicians' citizenship, which led to four more resignations from Parliament in May.
The by-elections for those seats will be held on July 28, along with another by-election sparked by Tim Hammond's decision to vacate the seat of Perth for family reasons.
