Two Canberra police officers have gone on trial in the ACT Supreme Court, one charged with assault and the other with helping his colleague with a cover-up.

Wade Joseph Howell, 37, is charged with assault causing grievous bodily harm, choking and attempting to pervert the course of justice.

Andrew Charles Smith, 46, has been accused of helping in the cover-up by arresting the alleged victim, rather than his friend, after the incident in April 2016.

Prosecutor Kylie Weston-Sheuber told the court Mr Howell had followed a woman out of the Canberra nightclub Mooseheads in the early hours of the morning, striking up a conversation with her.

She told the jury the woman's boyfriend followed the pair, until he came upon them in a park.

The couple had been visiting from Melbourne.

Today the alleged victim said that as he approached his girlfriend, she told him not to hit the other man because he was a police officer.

He said he was then taken aback when the man pushed him and told the police officer: "What the f**k, I was just going towards my girlfriend."

"The next thing I know he punches [me]. I fell," he said.

The man told the jury Mr Howell got on his back and held him down, while he continued to punch him.

"I couldn't do anything because he was so heavy," he said.

"Then he had his two arms around me and he tried to choke me.

"He said 'I am a police officer I will kill you'.

"I said 'Man I can't breathe, please let me go'."

The man said he eventually blacked out.

He said his girlfriend left and in the end Mr Howell helped him find his hotel.

Officer punched man once in self-defence: lawyer

Ms Weston-Shreuber told the court the police were eventually called, with Mr Smith arriving first, but instead of arresting Mr Howell the alleged victim was taken into custody.

She said the case arose after concerns raised by other police who went to the hotel.

Mr Howell's lawyer James Sabharwal told the court his client only punched the alleged victim once, and it was in self-defence.

He said his client also denied any cover-up.

Mr Smith's lawyer Steven Whybrow also said there was nothing untoward in his client's actions.

He said Mr Smith was charged with attempting to pervert the course of justice when he was doing his job.

The court is expected to hear from 20 witnesses over the 10-day trial.