A Sydney court has found police used excessive force against a teenager in a violent arrest at last year's Mardi Gras.

Jamie Jackson Reed, 18, was accused of assaulting two officers but police have withdrawn all charges against him.

Magistrate Michael Barko told Sydney's Downing Centre Local Court that the teenager was "brutalised" by a constable who slammed his head into the pavement.

Charges against the teenager were dropped two weeks ago.

"What should I infer?" Mr Barko said. "That the Commissioner wants no further bad publicity?"

The same officer, Constable Leon Mixios, was the subject of a Police Integrity Commission investigation last year, after a separate incident involving excessive force during an arrest.

Jamie Jackson Reed, pictured, before last year's Mardi Gras celebrations. Video footage of his arrest went viral, sparking allegations of police brutality. ( AAP/Facebook )

Mobile phone video footage of the arrest was shown to the court.

The footage, which was taken by onlookers, appears to show an officer throwing a shirtless man on the ground and stepping on his back.

It went viral online after the incident in March 2013, sparking claims of police brutality.

Mr Reed's lawyer Chris Murphy says his client was awarded legal costs and would consider suing for damages.

"Under new mandatory sentencing laws, the magistrate pointed out he could have faced up to five years' jail," Mr Murphy said.

"Courts tend to believe police officers and that might well have happened to him.

"At 18 years of age in the jail system, his life would have been miserable and different forever.

"He's been under a lot of stress. He's been totally exonerated today."

Police say they are conducting separate internal investigations into the two arrests.

In a statement, they say Constable Mixios is on office duties at Fairfield Local Area Command pending the outcome of the investigations.