NSW Police say an officer who was filmed repeatedly punching a man in the back of the head while arresting him over an alleged knife threat used an "approved manoeuvre".
A six-second video uploaded to social media showed the arrest taking place on the platform of Cabramatta train station on Monday night.
In the vision, a male officer can be seen with his knee on the man's back as he strikes the man at least three times to the back of the head.
The arrested man's hands appear to be underneath his body when he is being punched by the officer before he moves them above his head.
In a statement, NSW Police said they were aware of the video circulating online and said it only captured an "isolated moment" of the situation officers faced.
Police said officers had initially been called to Canley Vale train station where a 21-year-old man had allegedly been acting aggressively towards female commuters while armed with a knife.
They allege the man then assaulted a 46-year-old man who tried to intervene before he boarded a train and travelled to Cabramatta station where he allegedly threatened more commuters.
It is alleged the man was located a short time later and ran away from officers who repeatedly directed him to show his hands to confirm reports he had been in possession of a knife.
Video of incident 'very concerning'
When the man allegedly failed to comply, police said the officer used "an approved manoeuvre to affect the arrest".
The 21-year-old was subsequently searched, and no knife was located but police said they were canvassing the rail corridor for evidence.
Senior solicitor for police powers and administrative law at Redfern Legal Centre, Grace Cameron Lee, told the ABC the actions in the video constituted excessive force.
"On the face of that video, it is excessive force and it is really very concerning considering I guess how dangerous it is to strike somebody in the head,"
she said.
NSW Police said while the 21-year-old was being taken to a police vehicle, he allegedly assaulted an officer and later spat at an officer while at the station.
The man was taken to Liverpool Hospital for assessment, where he allegedly assaulted police again.
NSW Police said body-worn vision captured by the arresting officers had been reviewed by the commander and inquiries were ongoing.
On Tuesday, he was released from hospital and charged by police with 10 offences including assault occasioning actual bodily harm, assaulting police and hindering or resisting police in execution of their duty.
