The head of the juvenile prison system in the Northern Territory vigorously defended the use of a restraint chair on teenager Dylan Voller as long ago as September last year.

Key points: Ken Middlebrook told ABC in September 2015 he had seen footage of Dylan Voller hooded, strapped to chair

Ken Middlebrook told ABC in September 2015 he had seen footage of Dylan Voller hooded, strapped to chair At the time said he was satisfied Mr Voller had been treated fairly

At the time said he was satisfied Mr Voller had been treated fairly Initially denied Mr Voller had been left in chair for hours

Ken Middlebrook, then the commissioner of corrections, told the ABC 10 months ago that 17-year-old Voller had been "hooded" and tied to the chair because he was "threatening self-harm" and "playing up".

Voller, who was a detainee at the Youth Detention Centre in Alice Springs, was strapped into the chair for almost two hours at the Alice Springs adult prison.

Mr Middlebrook was quizzed about the incident last year in an on-camera interview with the ABC.

ABC News had been told about the incident but had at that stage been unable to obtain the actual vision shown last night on Four Corners.

Mr Middlebrook told the ABC Mr Voller had been wearing a "spittal hood" because he was spitting on staff.

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He revealed he had been aware of the incident because "it was reported" to him through the usual channels.

Asked what he had done about it he said: "I just made sure that everything that was done was supposed to be done and that they recorded the incident.

"And I was satisfied with that. I've got a lot of confidence in the staff and the superintendent that runs the centre and the senior staff there.

"I'm very confident they do the right thing and I asked them to check those things and they usually tell me straight back very quickly and I'm satisfied with that."

Asked if he was satisfied that Mr Voller had been "treated fairly", Mr Middlebrook replied: "Yes I am."

He initially denied that Mr Voller had been left in the chair for between two-and-a-half and three hours.

"I don't think that would be right," he said.

"I've not seen any evidence of that."

Asked how long the teenager had been left hooded and strapped to the chair he said: "It was for short periods of time."

After the interview finished, he consulted Corrections Department records and corrected his statement, telling the ABC Mr Voller had in fact been in the chair for one hour and 50 minutes.

"And that young fellow was threatening self-harm. And was doing himself some injuries. He threatened to smash his elbow against the wall and threatened to do some self harm," the statement said.

Asked if the level of force used on the prisoner was legal he replied: "Look, it's about protecting that young person. And you know our obligations are [to] make sure they're kept safe. If they're self-harming we need to take what steps we can to keep them safe."