Prahran MP Sam Hibbins on Friday announced he was quitting the Greens and would sit as an independent after having a "consensual relationship with a staff member, which breached party rules".
Mr Hibbins said he left the party with "goodwill" towards his colleagues.
But leader Ellen Sandell spoke out against her former co-deputy, expressing anger that she had been left to deal with the fallout of his actions which she revealed had been the subject of a complaint.
Sam Hibbins has quit the Greens after admitting he breached party rules.
She said the Department of Parliamentary Services had informed her on Tuesday about an allegedly serious complaint against Mr Hibbins involving inappropriate behaviour towards a staff member.
"He is no longer a member of our party room, and he will never be welcomed back as a member for our party room," Ms Sandell told reporters.
"He does not meet the standards of behaviour and expectations that I expect and my whole team expects ... I don't think he meets the standards of behaviour expected of any MP."
Ms Sandell said her understanding was that Mr Hibbins had confirmed some allegations and contested other claims.
She said the staff member was "very distressed" and was being supported by the party.
Mr Hibbins was contacted for comment.
In his earlier statement, he said he deeply regretted his actions.
"I recognise this was a significant error of judgement and understand the distress this has caused," he wrote on social media.
Ms Sandell said the revelation had left her "angry".
"This is a problem with men's behaviour across society, across institutions, including here, and it is too often that women are the ones picking up the pieces," the Greens leader said.
"The premier's had to do it, I'm doing it here today."
Jacinta Allan has dumped two Labor MPs, Will Fowles and Darren Cheeseman, this parliamentary term for past instances of alleged misconduct.
"To think, that a Parliamentary Integrity Commissioner might have prevented some of this behaviour is a very good question," Ms Sandell said.
The long-delayed bill to introduce a commission to investigate allegations against MPs of corruption and misconduct will come into place at the end of this year.
But while the legislation will offer an official mechanism for any complaint to be handled and investigated, the body will not have retrospective powers.
Greens leader Ellen Sandell said Sam Hibbins failed the standards of behaviour expected of any MP. (Caitlin Powell/AAP PHOTOS)
In October, laws setting up an independent Parliamentary Standards Commission to enforce behavioural standards and investigate workplace conduct passed the lower house.
Politicians found to have breached the code of conduct could face fines of between two and five per cent of their base salary, or be suspended from parliament.
Federal Greens leader Adam Bandt said he supported Ms Sandell and the partyroom's "immediate and decisive action".
Mr Hibbins, who is married with two children, said he was aware his announcement was a distraction from his work with the Greens but would "remain committed to serving (his) electorate of Prahran".
"I accept the consequences of my decisions," he said.
"I am working to make amends to the most important people in my life, my family and our community."
Department of Parliamentary Services secretary Trish Burrows did not confirm whether the office was investigating a complaint against Mr Hibbins, but said she took her obligations to all staff seriously.
"All workplace issues raised with DPS are managed appropriately with the wellbeing of staff at the centre, this includes fulfilling our privacy obligations in relation to staff," she told AAP.
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