Updating an earlier joint statement with the Prime Minister, Senator Brandis on Tuesday night said the inquiry's terms of reference would be "broad enough to examine the abuses and practices across the juvenile detention system in the Northern Territory".

But he resisted calls to probe the issue nationally, arguing the NT was "the particular problem that has been exposed" and that royal commissions of inquiry needed to be "focused" in order to yield results.

Senator Brandis told the ABC's 7.30 program he had discussed the draft terms of reference with the PM and would take that document to a scheduled meeting of the federal cabinet on Thursday.

Footage of children held in a behavioural management unit being tear-gassed, as well as another minor, Dylan Voller, being strapped to a mechanical chair and covered with a hood, prompted national outrage after it was aired on the ABC's Four Corners on Monday night.

Prime Minister Malcolm Turnbull immediately announced a royal commission, which in a joint statement released at 3pm Tuesday was described as focusing solely on the failures of the Don Dale centre in Darwin's east.

But by evening the Attorney-General indicated the terms of reference had been expanded to cover the whole juvenile justice system in the Northern Territory, and would investigate whether the examples exposed at Don Dale were part of a systemic problem in the Top End.

"The commission will have terms of reference sufficiently wide and sufficiently penetrating to get to the bottom of the conduct that was revealed last night and the broader question of the extent to which it is systemic conduct in the Northern Territory system, and the reasons why it was allowed to exist and to become a practice that was regarded as unremarkable by the officers who we saw in the vision," Senator Brandis said.

The Don Dale youth detention centre would still be the "principal focus" of the inquiry, he said. But it would not extend beyond the NT's borders because of a desire to keep the inquiry tightly focused.

"We don't propose to take it beyond the Northern Territory because that's the particular problem that has been exposed," Senator Brandis told 7.30.

"One of the things about these royal commissions is if they're going to be useful, if they're going to provide practical solutions to real and urgent problems, they should be focused."

Senator Brandis also defended his frontbench colleague Nigel Scullion, the Indigenous Affairs Minister, who admitted that earlier reports of misconduct at the Don Dale centre had not "piqued [his] interest sufficiently" to warrant further investigation.

"There wouldn't be a person I can think of with a deeper concern in that portfolio for the wellbeing of Indigenous people - and a really practical common sense appreciation of the needs of Indigenous communities - than Nigel Scullion," the Attorney-General said.