The UN has delivered a scathing assessment of Australia's treatment of prisoners, immigration detainees and juvenile offenders, despite attempts to block inspectors from local facilities.

The UN subcommittee on prevention of torture attempted to visit Australian prisons in October 2022, after the federal government ratified the Optional Protocol to the Convention Against Torture five years earlier.

But the trip was delayed after UN inspectors were denied full access to jails and detention facilities in NSW and Queensland.

Its report noted the "discourteous, and in some cases hostile" reception from government officials and "persistent negative media coverage ... amounting to what the subcommittee would qualify as a smear campaign".

"Such behaviour also raises concerns (as it is assumed to be reflective) of how persons deprived of liberty are treated," the report found.

Inspectors found handcuffs, belt tethers and shackles were routinely used on children without consideration of whether they were necessary, while spit hoods were still applied in many facilities.

"The subcommittee considers that the use of both spit hoods and spit guards constitutes a form of inhuman and degrading treatment, which in some cases may amount to torture," the report said.

Up to 70 per cent of the prison population in some centres, alongside one per cent of the entire Northern Territory population, were people held in custody while awaiting the start of trials or sentence hearings.

The report also noted an "excessive use of surveillance" in every location inspectors visited, and observed one juvenile detention centre where showers did not have screens and instead faced common areas in full view of detainees and guards.

At the Banksia Hill Detention Centre in WA, children as young as 10 years old slept on mattresses on the floor and had no access to running water.

Sometimes, they were left alone in their cells for up to 23 hours a day, amounting to "de facto solitary confinement".

The committee said interviews with staff at the Don Dale Youth Detention Centre in the NT highlighted a gradual shift in their functions from social workers to role more akin to those of prison guards.

It said access to health care was also abysmal with prisoners experiencing excessive wait times for medical or psychological assistance.

In one instance, a woman who suffered a fractured skull, cerebral contusion and other injuries after falling in a detention facility did not receive any further treatment or monitoring after being discharged from hospital.

The immigration system was no better with migrants subject to visa cancellation despite arriving in Australia as children, and vermin running rife in detention centres.

The report's final recommendations included raising the age of criminal responsibility to 14, banning spit hoods, ending strip searches, providing a mechanism to review decisions made by the immigration minister.

The federal government's 58-page long response, also published on Wednesday night, does not commit to any of the UN's suggestions.

It instead provides an extensive list of policy positions already in place, and says the government would "provide the subcommittee with updates on Australia's progress and looks forward to continued engagement".

It also rejects calls for overarching federal legislation even though many issues highlighted in the report were caused by discrepancies between states and territories.

Public Interest Advocacy Centre principal solicitor Jonathan Hall Spence said the report noted a "troubling" use of disproportionate force.

"Australia presents itself as a good global citizen, but this report shows we are failing to meet basic obligations to protect the human rights of people in prisons, youth detention and immigration detention," he said.

Greens senator David Shoebridge accused the federal government of failing to meet their obligations in ensuring compliance.

Australian Associated Press