Activists are calling on the Victorian government to implement the United Nations' anti-torture protocol so children in custody are kept safe.
The Commonwealth government in 2017 signed on to the international convention, which requires independent oversight bodies to inspect all places of detention.
It was up to individual states and territories to implement the protocol but Victoria has failed to do so, the open letter published on Friday says.
"Victoria has now missed multiple deadlines for implementation of the bare minimum safeguards to protect against mistreatment in prison," the letter says.
"The impact of this failure ... is acutely felt by children locked away in Victoria's youth prisons."
The Victorian Aboriginal Legal Service, the Human Rights Law Centre and Jesuit Social Services are among the organisations to sign the open letter.
They say immediate action is needed following repeated reports of mistreatment of children in custody.
It follows a report from the Commission for Children and Young People, which revealed a 17-year-old boy was placed under a spit hood while in adult prison.
The boy also had the water in his cell turned off for about 22 hours, meaning he could not use the toilet or wash his hands.
The letter calls on the state to urgently implement the UN protocol so prisons and youth justice centres are monitored for human rights abuses.
"Rather than funnelling money into prisons, the Victorian government must fund long overdue independent oversight of them, alongside community-led responses to divert children away from prisons in the first place," the letter says.
The Victorian government said there were strong protocols in place to ensure children's human rights were protected in custody.
"Hardworking youth justice staff undertake significant training to manage difficult situations and the work they do supports better rehabilitative outcomes (and) keeps young people in custody safe," a spokeswoman told AAP.
The state government also criticised the federal government for not providing ongoing funding to the states to implement the protocol, despite being the ones to sign on to it.
In the Northern Territory, Don Dale youth detention centre remains open more than five years after a royal commission recommended its closure.
With an election due in August, NT Attorney-General Chansey Paech says he remains committed to closing the facility.
"It's anticipated we'll get on with the job and get that done," he told reporters on Friday.
A replacement facility was due to open in 2023 but the government blames building approvals, staffing and COVID-19 for the delay.
An inquest into the death of a Mayali father while being held in a Darwin prison has also sparked calls for better carceral health services in the territory after experts testified his death was preventable.
Mr Dooley suffered a heart attack and died several days later in hospital due to irreparable heart damage.
