South Australian co-leader Tammy Franks - a Greens MP for 15 years - said she would sit on the crossbench from Tuesday as a "progressive independent" until the 2026 state election.
"I still believe in a Greens' vision for a better world and know there are many, many wonderful, committed and well intentioned people within the party," she said.
"But not all have such noble motivations and they and their scheming are being too often enabled."
Ms Franks had announced in 2024 she would retire at the next election, but on Tuesday she said she "wouldn't rule anything out".
"I cannot properly do the job I was elected to do until March 2026 … when trust has been broken, and I spend more time watching my back than thinking ahead," she said.
"A small coterie colluded against me hoping to sabotage my work, to wear me down and dissuade me from continuing."
She said she had been accused of misconduct by this group and "gaslit and subjected to a kangaroo court, all masterminded by a staffer I am now meant to work with".
"The Greens' processes should recognise the often mercenary tactics used by those who seek control, power and promotion," she said.
She said the "ringleader" was employed in the office of the Greens' other upper house MP, Robert Simms, "and I do not feel safe in the situation where we are meant to work together".
In a statement, Mr Simms did not address Ms Franks' allegations but said he was "deeply saddened" by her decision.
"Tammy has been rightly lauded for her work on a range of important portfolio areas and been a strong voice for the Greens over the last 15 years," he said.
"We have worked very closely together during this time.
"I know many members will share my sadness at this outcome, but our movement is bigger than any one person or individual."
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