The teenager placed in a mechanical restraint was put there at least three times before the controversial Four Corners footage was shot, a witness to the abuse says.
The new claim jars with the version of events from the Northern Territory Government, which insists the restraint was only used on Dylan Voller once.
Four Corners showed images on Monday night of Voller hooded and strapped into a mechanical restraint chair for almost two hours in March 2015, when he was serving a two years and three months minimum sentence.
Voller, now 19, told his lawyers he had been put in the chair on several occasions, but the new information from the guard is the first independent account.
Ben Kelleher, a former youth justice officer, said: "I know of three times he was in the restraint chair."
Each time he was locked in the restraints he was "never so still, never so sheepish as he was when he was in that chair".
"I turned up for one shift and Dylan was in the chair and the other two times they were on incident reports I read once I got to work," he told the ABC.
He worked in youth detention centres between 2011 and 2014. At first he didn't think strapping Voller to the chair was wrong - but now he does.
"I truly believe no matter how misbehaved a young man or lady is, they shouldn't have their right to movement taken away like that.
"It's very hard for the individual and a worker to draw a line between your safety and the safety of the kids."
The official response from the Northern Territory Corrections Department is that it was used "only at adult correctional centres", and youth detainees had been placed in it just a single time.
Voller's lawyer Peter O'Brien is calling for his immediate release and said his client was
"scared for his safety". He has also tweeted about suing the NT Government on behalf of Voller.
"Assault, battery and false imprisonment. It's a shame we can't sue for permanent psychological damage," Mr O'Brien tweeted.
Voller released a letter after the Four Corners episode aired, thanking "the whole Australian community" for its support.
"I would also like to take this opportunity to apologise to the community for my wrongs and I can't wait to get out and make up for them," he wrote.
On Tuesday afternoon, the NT's Chief Minister Adam Giles announced he had removed John Elferink as Corrections Minister, installing himself in the role. The move came after Prime Minister Malcolm Turnbull announced a Royal Commission into the treatment of children in juvenile justice centres in the NT.