More than 700 people rallied in Melbourne on Saturday, with similar protests held in other major cities around the country in response to grisly video of aboriginal children being physically abused and humiliated by prison guards in the Northern Territory.

Prime Minister Malcolm Turnbull has ordered a state inquiry after the Australian Broadcasting Corporation aired footage this week showing guards at the Don Dale youth detention center tear gassing teenage inmates and strapping a half-naked, hooded boy to a chair.

UN Special Rapporteur on Torture Juan Mendez said that the use of hoods, restraints and gas on children in detention centers could violate the UN treaty barring torture.

Rupert Colville, a spokesman for the UN Office of the High Commissioner for Human Rights, added: "We are shocked by the video footage that has emerged from Don Dale youth detention center in the Northern Territory."

"Some children were held in isolation for extended periods, sometimes for several weeks, in hot and dark cells with no access to fresh air or running water," Colville said. "In one incident, six children were teargassed by prison guards."

But, despite near universal condemnation by international rights bodies, Turnbull has rejected calls for a broader inquiry that would cover treatment of youth detainees across Australia.

"If you spread it out to be an all-Australia inquiry, it would go on for years and you won't get the answers you need in respect to the Northern Territory," Turnbull told the ABC earlier this week.

At Saturday's rally, that was rejected out of hand by protesters angry at the government's perceived foot-dragging.

"The elephant in the room is that it is a racism problem, but they aren't addressing that," indigenous Australian rapper Adam Briggs told the Reuters news agency.

"The [Northern Territory] government ... is part of the problem and cannot be trusted to investigate themselves," Aboriginal and Torres Strait Islander lawyer Matthew Bonson told the Australian Associated Press.

The abuse case highlights concern about the disproportionate numbers of aboriginal youth in custody, with indigenous leaders calling for politicians to deal with the wider issue of the treatment of Aborigines.

Aborigines compose just 3 percent of Australia's population but make up 27 percent of its prison population; in the Northern Territory, they represent 94 percent of juvenile inmates.

Roughly 700,000 indigenous citizens track near the bottom of almost every economic and social indicator for Australia's 23 million people.

jar/jlw (Reuters, AAP)