A Ballarat magistrate has found a police sergeant who chased his former neighbour from his house and punched her in the face did so in self-defence.

Key points: David Berry has been acquitted of assault charges

David Berry has been acquitted of assault charges Mr Berry said he struck Samantha Mitchell because she was trespassing on his property

Mr Berry said he struck Samantha Mitchell because she was trespassing on his property Magistrate Frank Jones said the credibility of Ms Mitchell was an issue

Sergeant David Berry pleaded not guilty after he was charged with common assault last year, arguing he struck his former neighbour when he was off-duty because he feared "she was about to hit him".

At a hearing in Ballarat, Magistrate Frank Jones accepted Mr Berry's argument and said the case centred around the credibility of the complainant.

The magistrate said Ms Mitchell, 34, was "loose with the truth", had "a serious drinking problem" and "a pathological hatred of Ballarat police".

He said Mr Berry had "no axe to grind" with his former neighbour, and that "in contrast" with the 34-year-old, he found Mr Berry's evidence "believable".

Mr Jones said Mr Berry's response, "as he perceived it, as [she] raised her arms" on his approach in the darkened street, was "reasonable in the circumstances".

Ballarat police sergeant David Berry said he struck Samantha Mitchell in "self defence". ( Facebook )

"He only had his hands to defend himself with," the magistrate said.

In evidence given as part of a contested hearing last month, Mr Berry said he was not aiming at "any specific area" when he hit her.

Ms Mitchell fell after she was punched and her nose was broken in two places.

She ultimately required surgery to fix it, with bruising to her head and eye.

She told the court she had knocked on the officer's door during a dispute with her partner on the night but fled from the property when Mr Berry reacted angrily and swore at her.

But Mr Berry disputed that he was ever angry.

"I wasn't furious, I'd just woken up," Mr Berry said.

Off-duty officer chased neighbour to 'arrest her'

Ms Mitchell gave evidence that she was punched to the back of the head as she ran down the street away from the off-duty officer.

However, the court accepted evidence from Mr Berry, who said he punched her to the side of the head face-on when she stopped running and "lunged at him".

Ms Mitchell's injuries required surgery. ( Supplied )

"Why did you chase her?" the prosecutor asked.

"I wanted to know what was going on," Mr Berry said. "My intention was to apprehend or arrest her."

Mr Berry acknowledged he had no legal grounds for making a citizen's arrest, given there was no risk to public safety or order, but said he "had every right" to find out why Ms Mitchell had been "trespassing" on his property.

"It never got to that point, because she attacked me," said Mr Berry.

Mr Berry was questioned as to why he called Ballarat police, in place of triple-0 or an ambulance, after Ms Mitchell fell to the concrete and hit her head.

The officer said he "didn't see blood" but acknowledged to the court that, in hindsight, he would ask if she was OK if he was in the same situation again.

Neighbour 'disappointed' with decision

Ms Mitchell, a former police officer from Queensland who is now studying law, sobbed inconsolably at different points while giving evidence in April.

She said that she was in "tremendous pain" as a result of the punch, and felt like she had been hit with a sledgehammer.

"It's a powerless situation and it's devastating to be in," she said in her evidence.

After the hearing, Ms Mitchell released a statement saying she was disappointed at the court's decision and the "psychological impact" of the court process had been "truly more harmful" than her physical injuries.

"My reputation has been destroyed and my self-esteem crushed," she said.