Geneva: Australia's detention of refugees, including children, is 'cruel, inhuman or degrading treatment' forbidden by international law, a United Nations report has found.

The report released on Friday in Geneva by the UN Committee Against Torture called on Australia to stop putting asylum seekers into mandatory detention, and to make sure that asylum seekers on Manus Island and Nauru are treated more humanely, and their claims are promptly and properly assessed.

"The combination of … harsh conditions, the protracted periods of closed detention and uncertainty about the future reportedly creates serious physical and mental pain and suffering," the report said.

In written observations the committee said Australia should repeal the laws that send all 'irregular' arrivals into mandatory detention.

Under the Convention Against Torture, Australia must prevent torture and other acts of cruel, inhuman or degrading treatment or punishment when people are imprisoned or detained.

Claudio Grossman, chair of the ten-person committee and the "rapporteur" who investigated Australia's treatment of asylum seekers, told Fairfax that during his investigation Australia had not provided him with evidence of its compliance with this convention.

He said Australia had failed to show that it was not sending asylum seekers back to countries where they faced a "substantial risk of torture".

He added it was clear that Australia had "effective control" over the detention centres in PNG and Nauru, and so it was responsible for ensuring that they complied with Australia's obligations under the convention.

"When there is mandatory detention of undocumented immigrants and children, that runs counter to our interpretation of the convention," he said.

The committee said it was "concerned that detention continues to be mandatory for all unauthorised arrivals, including for children," in the report compiled after hearing evidence from human rights groups as well as the Australian government.

"Detention should be only applied as a last resort," the report said, only when "strictly necessary" in each individual case, and should be for as short a time as possible.

It said it was concerned at Australia's policy of transferring asylum seekers to processing centres in Papua New Guinea and Nauru, "despite reports on the harsh conditions prevailing in these centres, including … overcrowding, inadequate health care and even allegations of sexual abuse and ill-treatment".

The Committee was concerned "in particular [about] the policy of intercepting and turning back boats, without due consideration of [Australia's] obligations" under international law.

Anyone who arrives or attempts to arrive in Australia seeking asylum or protection should be guaranteed that their claims are thoroughly examined, and be able to challenge any adverse decision.

"[Australia] should continue and redouble its efforts" to find an alternative to closed immigration detention.

Mr Grossman said the committee was also concerned by the high proportion of indigenous people in jails, and the situation of women in detention facilities, particularly indigenous women.

The committee also welcomed the Royal Commission into child sexual abuse, however it said it was "concerned" as to "whether the outcome of its work will result in criminal investigations, prosecutions and compensation for victims".

It also:

• - criticised the treatment of inmates at Roebourne Regional Prison
• - recommended the Australian government consider abolishing the use of tasers
• - expressed its 'concern' over Australia's counter-terrorism legislation, including the "broad" definition of a terrorist act, and the detention powers of ASIO.

Fairfax asked if an Australian government spokesperson was available to respond to the committee's report, but has not yet received a reply.

In its appearance before the committee earlier this month, Australia's delegation said it "takes its obligations under the Convention very seriously. Since ratifying the Convention in 1989, Australia has worked to ensure Australia's laws, policies and practices are consistent with our international obligations."

Australia's permanent representative to the UN John Quinn told the committee that the government had "striven to improve the design and procedures of its migration programmes to enhance fairness, accountability and integrity.

"A robust returns process for dealing with those found not to be in need of protection is fundamental to the integrity of status determination processes."