A Northern Territory police investigator says she thought a youth justice officer was playing a game when he allegedly hit a detainee's kneecaps with a torch, a royal commission has heard.

Where is the royal commission up to? Hearings on the management of detention centres have wrapped up

Hearings on the management of detention centres have wrapped up Commission now hearing evidence on issues pre and post detention

Commission now hearing evidence on issues pre and post detention Next hearings will focus on child protection and alternatives to detention

Detective Sergeant Kirsten Engels gave evidence to the royal commission into youth detention and protection about her role investigating allegations of abuse made by a juvenile detainee.

The child, known as AT, alleged that on one occasion former youth justice officer Conan Zamolo hit him repeatedly on his kneecaps with a torch and on another had filmed detainees masturbating in the shower.

Counsel assisting the commission, Peter Morrissey SC, recounted AT's statement that he had been hit three times while he was drinking water before trying to run to the dining room, and then when he returned to his room Mr Zamolo continued to hit him as AT curled into a ball.

Mr Morrissey: There's no game you know of that involves that, is there? The game of hitting with a torch? Detective Sergeant Engels: Yes, I believe there can be games that occur ... yes I can. Mr Morrissey: A game between a guard and a mid-teenage boy hitting with a torch?

Detective Segeant Engels said when she asked AT later why he thought Mr Zamolo had used the torch, AT had said he thought Mr Zamolo thought it was a game.

The commission heard when AT ran away from Mr Zamolo after the first torch hits he had gone to the dining room "cause there was (sic) cameras there ... cause they were going to belt me, I know".

It also heard AT was "clotheslined" by a separate guard while trying to run through the dining room, before returning to his room and Mr Zamolo.

Zamolo never charged over incident

Ms Engels said did not think police would be able to successfully prosecute Mr Zamolo. ( Supplied )

When Mr Morrissey asked Detective Sergeant Engels why she had not questioned Mr Zamolo about the incident or laid charges, she said she and other colleagues had deemed there was no intention to assault AT.

"We had built a bit of a profile on Mr Zamolo ... it seemed feasible knowing what we knew about him," she said.

"His maturity level was only one notch above the children he was working with.

"It seemed feasible that there was some sort of behaviour going on that he felt that he wanted to be on the level with those boys, goofing around, joking around with them all the time."

She said she and her team collectively agreed the behaviour he was displaying was "not OK" but did not pursue charges against him.

The commission also heard Detective Sergeant Engels had been one of a line of investigators looking into the allegations Mr Zamolo filmed detainees on Snapchat in the shower, for which he also was not charged.

"It seemed highly unlikely that [a prosecution] would be anywhere near successful given we didn't have that, we didn't have the actual image to put before the court," she said.

"But you had two witnesses saying they'd seen the image on the phone, correct?" Mr Morrissey asked.

"Yes, however, in my experience with the criminal courts and the level of proof we have to bare, to not have the actual image in front of them seemed to me [a prosecution would be] incredibly unlikely," she said.

The hearings continue.