The senior constable, who has since left the area but remains a serving police officer, was charged with assault occasioning actual bodily harm, common assault and destroying a necklace by tugging it against the woman's neck.

The assault charges related to the officer pushing the woman's face into a police vehicle, including on one occasion when she was handcuffed and he was gripping her hair.

The senior constable said he was concerned the woman may have had infectious diseases such as hepatitis C or HIV and he saw saliva come from her mouth when she was yelling.

He said he had an open cut on his knuckle, caused by a brake leaver on her bike, so he was concerned about contracting an infection and wanted to push her face away so he wouldn't be spat on.

The charges were all dismissed on the grounds of self defence or not exceeding reasonable force.

Magistrate David Day said the officer had a right to avoid being spat on but he had "a little bit of reluctance," in dismissing the damaged necklace.

"It probably could have been done better but that's in retrospect," Mr Day said.

The charges were made after fellow police officers reported the man's actions during the arrest in Lords Place after police saw the woman ride a bicycle through a red traffic light at 9.20pm on December 30, 2017.

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The police officer who was patrolling with the senior constable, four other responding officers, the officer in charge, and a police trainer, were called as prosecution witnesses.

Despite a subpoena, the woman, who was previously convicted for her actions, did not appear in court and refused previous requests for her statement.

The main arresting officer said the senior constable struck the woman in the side of her face when she was pinned against the side of the police vehicle.

However under cross examination he conceded it was possible the senior constable had just pushed her face away.

He and other witnesses also said that after the woman was handcuffed the senior constable grabbed her by her hair and pushed her face into the police vehicle.

The senior constable said he did it because she moved her head back and he didn't intend to push her face into the vehicle.

That incident led to the charge of actual bodily harm due to facial injuries the woman was treated for, but it could not be proved beyond reasonable doubt that the injuries were caused by the officer's actions.

However, the defence presented a statement by from a custody officer, who spoke to the woman and recorded the injury as being caused by her bike.

The officer said he ripped the necklace off in two tugs because it needed to be removed for transportation, however he said it had saliva on it and he wasn't wearing gloves so wanted to touch it as little as possible.