Gellung Warl, Victoria's First Peoples' representative body, sent Corrections Minister Ros Spence, Attorney-General Sonya Kilkenny and First Peoples Minister Gabrielle Williams a letter on Tuesday calling for the legislation's immediate withdrawal.
Under the bill, a prisoner's right to an hour of open air each day would become subject to "operational considerations".
These include inmate safety and prison management and would add to a rule allowing outdoor access to be cancelled due to poor weather.
The bill passed the lower house in June with bipartisan support and is expected to be debated in the upper house, where Labor has the numbers to pass it with coalition support.
The letter, signed by First Peoples' Assembly co-chairs Meriki Onus and Djaran Murray-Jackson, declared the legislation was "inconsistent with the spirit and intent of statewide treaty".
"It undermines the renewed relationship between First Peoples and the state agreed to under statewide treaty," they wrote.
It is the first time the assembly has used the new submission power under treaty, which requires the minister to respond in writing within 60 days.
Human rights and legal groups, including the Human Rights Law Centre, Victorian Aboriginal Legal Service, Fitzroy Legal Service and Jesuit Social Services, have also called for the bill to be scrapped.
They were expected to join a protest against the legislation outside parliament on Wednesday afternoon.
The Victorian government itself has admitted the reforms may be incompatible with elements of the state's human rights charter and partly incompatible with treaty.
"Aboriginal people may be disproportionately affected by the bill, particularly those who have been separated in prison," then-corrections minister Paul Hamer told parliament in June.
The move was necessary for the safe management of open air access and prison function, Mr Hamer said.
Kelly Flanagan, 39, spent five months in solitary confinement before being released from Melbourne custody in early 2025 after serving three-and-a-half years for offences including kidnapping.
The Wiradjuri woman said her isolation, and occasional restrictions on her access to open air, has undermined her mental health and set back her rehabilitation prospects.
"You can't expect people to be good citizens when they are released if you treat them like animals in prison," she told AAP.
The bill was developed in response to international crime boss George Marrogi's Supreme Court case against Victoria's Department of Justice in January.
Marrogi, who is serving a 38-year sentence for murder and drug trafficking, successfully argued his human rights had been breached over his limited access to open air in custody.
Victoria's justice department is appealing the ruling, with a hearing set for October 23 and a cross-appeal filed by Marrogi.Â
The bill would also limit the government's financial liability arising from the judgment.
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