A Victorian police officer who punched a teenager in the face "as hard as he could" has been found guilty of assault and attempting to pervert the course of justice over the incident, which was filmed by the boy's friend on his mobile phone.

Senior Constable Simon Mareangareu and Leading Senior Constable Dennis Gundrill approached 17-year-old Kyan Foster and his friend, Stuart Laird, 16, as they walked home through Melbourne's east in the early hours of Christmas Day in 2014.

The jury was told Mr Foster was giving his details to Mareangareu when the police office punched him in the face after accusing the teenager of being a "smart arse".

Mareangareu told the court he had punched Mr Foster "as hard as [he] could to the face", in line with his training, after the teenager "whacked" him in the arm.

Simon Mareangareu had pleaded not guilty to the charges ( Supplied: Eddie Jim/The Age; file photo )

A jury took one day to find Mareangareu guilty of common law assault and attempting to pervert the course of justice, but not guilty of perjury or falsely imprisoning the teenagers who were arrested after the assault.

The officer was also found guilty of attempting to pervert the course of justice for having an improper conversation with Mr Foster's mother when he was serving the charges against her son.

Mr Laird captured the Christmas assault on his mobile phone and was told to "get rid of that video right now" by one of the officers, the court heard.

He claimed the video had been deleted when he was released from police custody on the night of the assault.

The jury was told Mr Laird's father paid experts a "significant sum of money" to have the video recovered.

But Mareangareu was cleared of the allegation that he had deleted the video of the incident.

The jury found Leading Senior Constable Gundrill not guilty on all charges, including perjury, false imprisonment and attempting to pervert the course of justice.

Mareangareu has been released on bail and will return to court next month for a plea hearing.

Police officer 'thought teens were planning robbery'

The court released video of the incident to the media but noted it was not complete, did not capture the whole incident, the video had been enhanced, and the audio was not properly synchronised with the vision.

In the footage, Mareangareu demands the teenagers' names and addresses, telling them they were suspected of committing burglaries.

He then grabs Mr Foster's jumper, after accusing him of being a "smart arse".

Mareangareu can then be seen swinging his arm towards the teenager and tackling him to the ground.

During the trial, Mareangareu gave evidence that he had stopped the boys after becoming suspicious they might be planning to rob a convenience store.

"There's two young blokes in hoodies with backpacks, across the road from 7-Eleven, so I'm thinking 'are these blokes about to rob the 7-Eleven?'," Mareangareu said.

"I'm starting to get a bit antsy and I'm thinking there's something else going on here and I need to find out what it is.

"I'm getting the picture now that maybe these blokes have got weapons … they might have a knife, gun, taser."

Prosecutor Bruce Walsmley QC accused the police officer of lying about being concerned the boys might have planned to rob the convenience store, but Mareangareu rejected that statement.