WA cop on trial over assault on recruit

An off-duty detective with a "hair-trigger temper" punched a police recruit outside a bar in an unprovoked attack, fracturing an eye socket, a Perth jury has been told.

Shane Michael Ferguson is on trial in the West Australian District Court, accused of causing grievous bodily harm to 19-year-old Josh Prime in July last year.

In his opening address, prosecutor Mark Hunter said the victim, who later graduated from the police academy, was out with friends at the 7th Ave Bar and Restaurant in Midland while Ferguson was attending a police social club function.

A third man had done a burnout in the street and was approached by Ferguson, who said he was a policeman.

But Ferguson was a stranger to the driver and his passengers, and his conduct had not convinced them he was a law enforcer, Mr Hunter said.

The prosecutor said the driver gave Ferguson "the bird", which set off Ferguson's "hair-trigger temper".

The detective then told the driver: "Mate, you're f***ed, I'm a cop."

An altercation began and Mr Prime, who was concerned about the safety of some of the women in the group, approached, telling Ferguson: "You're supposed to be a police officer."

As Mr Prime tried to take the women away from the conflict, Ferguson punched the victim, who hit his head on the footpath but did not lose consciousness, the court heard.

Mr Hunter said the attack was witnessed by several people who would testify what they saw.

The trial continues.