A New South Wales police officer has been found guilty of assaulting an Indigenous boy during an arrest in 2020.

On Monday Magistrate Rami Attia found constable Ryan Barlow guilty of occasioning actual bodily harm during the arrest at Ward Park in Sydney's inner-city suburb Surry Hills.

A video of the incident captured by the boy's friend, which was posted online, shows Constable Barlow holding the teen's arms behind his back and using a 'leg sweep' motion to slam him to the ground.

The video, alongside bodycam footage from officers, were central to the case.

The magistrate was unconvinced by claims Mr Barlow, who pleaded not-guilty to assault occasioning actual bodily harm, felt threatened by the teen who can be heard saying "I'll crack you across the jaw, bro" from four metres away while his friends were speaking to officers present immediately prior to his arrest.

It was found that Mr Barlow had given the young boy just 3.02 seconds to comply with demands before employing the leg sweep which saw him fall forwards and land heavily on the ground.

The boy's friend can be heard saying "you just slammed him on his face…he's in pain" in the video.

"The complainant in the video was clearly, audibly and physically in pain," the magistrate said.

In a court hearing last year, a police interview was played where Mr Barlow told colleagues the boy started to kick back, leading him to fear he was about to be assaulted.

Magistrate Attia said this contradicted what can been seen in the videos.

In the interview, Mr Barlow said he was aware of the complainant from a previous attempted shoplifting incident involving a replica firearm.

While the magistrate said the leg sweep was not part of police training it was not prohibited, and said in the instance in question it was not a reasonable amount of force, nor was it in self-defence.

"The risk of danger sought to be prevented was either minimal or non-existent at best," Magistrate Attia said, according to the ABC.

Emma Hearne, Associate Legal Director of the National Justice Project, who represented the boy assaulted by the officer, noted that "far too often, police misconduct goes unpunished".

"Today, our client has secured accountability for the brutal assault inflicted upon him. We congratulate the family for bravely standing up in court against the same institution that left their son in hospital," she said.

"Following today's positive decision, we will continue working with our client to ensure that police change their policies on use of force and de-escalation.

"Today's outcome not only represents justice for our client, but it sends a powerful warning to NSW Police that they need to change their practices. We will continue our work in holding police to account for misconduct against the community in order to drive a change in culture.

"I have no doubt many others suffer similarly from police brutality, and today's outcome is likely a result of the footage which provided clear evidence of what occurred. I hate to think what could have happened if bystander footage wasn't available."

Mr Barlow replied "no comment" when asked in his police interview if he would have done anything differently in hindsight.

He will be sentenced at a later date, according to reports.