The mistreatment of juvenile detainees in the Northern Territory could amount to torture and is "very worrisome", a UN expert on torture has said.

Four Corners this week aired footage showing teenagers being tear-gassed, physically assaulted, forcefully stripped naked, shackled and hooded in Territory prisons and youth detention centres between 2010 and 2015.

Juan Mendez, the UN Special Rapporteur on Torture and Other Cruel, Inhuman and Degrading Treatment or Punishment, said he had asked the Federal Government to provide information on how the incidents will be investigated.

"It's hard to tell only from the video or the press coverage but I do think that it's a very worrisome development that can amount to torture or cruel inhuman and degrading treatment under any circumstance," he told Radio National.

"Of course I am very concerned, and engaging with the Australian Government on the matter, hoping to get some answers."

Prime Minister Malcolm Turnbull has announced a royal commission into the NT's youth detention regime, but the terms of reference are yet to be announced.

Juan Mendez says those who participated or covered up the violence must be appropriately punished. ( Reuters: Dinuka Liyanawatte )

Mr Mendez said the PM's move was "encouraging", but added that he wanted to know the extent of the inquiry.

"States are responsible for violations of this sort, but if they do react properly then their international responsibility is not engaged, or at least it's improved," he said.

"We need to know … that everyone that participated in it, including those that may have covered it up, have been appropriately punished, reparations have been offered and paid to victims … all these things are part of Australia's international obligations."

Mr Mendez said the acts could potentially be classified as crimes under international law but more evidence was needed.

Following the Four Corners report NT Chief Minister Adam Giles removed John Elferink as NT Corrections Minister to take on the role himself.

Mr Giles said the NT would build a new youth detention facility, with planning to commence immediately.

In the wake of the alleged mistreatment, six prisoners who spent time at Don Dale Juvenile Detention Centre are now suing the NT Government for incidents that occurred while they were in detention.