Four Aboriginal men who were unlawfully subjected to tear gas while detained as children at the notorious Don Dale Youth Detention Centre have secured a landmark victory in the High Court, bringing a 12-year legal battle against the Northern Territory Government to a close.

The four men were exposed to CS fogger tear gas by a prison officer in 2014 after a violent incident at the Darwin facility. The gas was deployed during a confrontation between staff and another detainee, Jake Roper, whom officers sought to "temporarily incapacitate".

In 2020, the High Court ruled that Josiah Binsaris, Keiran Webster, Ethan Austral and Leroy O'Shea were entitled to damages after being exposed to the gas. A Northern Territory Supreme Court judge later awarded the four men $200,000 each in exemplary damages. However, the NT Court of Appeal reduced that award last year after the Northern Territory Government argued the amount was manifestly excessive.

NT failed in its duty of care

On Wednesday, the High Court overturned the Court of Appeal's decision, although it reduced the exemplary damages to $50,000.

"The Northern Territory had a duty to ensure the physical, psychological and emotional welfare and safe custody and protection of all detainees in the detention centre," the majority of the court found.

"Knowing that Don Dale was unfit for purpose...it was for the Northern Territory to provide adequate training to its officers to ensure they could respond lawfully and appropriately to serious misconduct by a detainee held in Don Dale.

"The Northern Territory manifestly failed to do so and, in so doing, is to be inferred as having also failed to ensure its officers could maintain their professional discipline in the face of provocative and intimidatory conduct by one detainee."

Mr Austral was awarded a total of $90,000 and Mr Binsaris $70,000, while Mr O'Shea and Mr Webster were each awarded a total of $100,000 each.

'I thought I was going to die'

On August 21, 2014, Mr Roper escaped from his cell within the Behaviour Management Unit and damaged property. Guards ultimately decided to use CS gas to regain control of the situation.

When discussing how to deal with Mr Roper, court documents show one of the guards saying: "Let the f**ker come through because while he's coming through, he'll be off balance. I'll pulverise the little f**ker. Oh shit, you're recording."

The four plaintiffs were exposed to the gas while locked inside their cells. The NT Supreme Court previously found they were assaulted, handcuffed and taken to a basketball court, where they were "hosed down to remove any residue of gas".

Court documents show the gas was deployed 10 times inside the unit, which had no windows, air conditioning or running water for drinking or washing hands. When asked if he intended to "gas the lot of them," the Director of Correctional Services replied: "I don't mind [sic] how much gas you use."

If used in large enough concentrations, CS gas "can cause serious injury or death," a training manual for officers said, noting it can cause "extreme burning of the eyes, irritation of the nose and throat, tightness of the chest, difficulty in breathing, possible fear of suffocation, dizziness, vertigo and nausea".

Mr O'Shea, who was asthmatic, shared a cell with Mr Webster. In his affidavit, he stated he feared he was going to die after the gas was deployed.

"The teargas affected me badly and from the way I saw Keiran acting, it seemed to affect him badly as well," he said.

"My throat was burning, I was choking, my eyes were stinging and my nose was running. Keiran looked like he was throwing up.

"After they sprayed the teargas, I felt complete fear. I thought I was going to die. The worst thing was not knowing how long it was going to last and how long we were going to have to sit in there and burn."

Mr Webster noted it "was so hard to breathe that Leroy and I thought we were going to die".

In a statement, the North Australian Aboriginal Justice Agency (NAAJA) welcomed the ruling, with Civil Managing Lawyer Andrew Roberts saying the decision "recognises the seriousness of the NT Government's conduct".

"At the end of the day, these were vulnerable children in the care of the state," he said. "The Government has to do better and must be accountable for its actions."

Mr Roberts said that, 12 years later, the government had failed to apologise and no officer had been held accountable.

"This is an incredibly important outcome for our clients, who have carried the weight of this with them for many years and who can now begin to heal and move forward."