Chunping Quan bit the waiter outside an Adelaide restaurant after she and her companion paid only half the price on the bill because they claimed they were served half a fish when they ordered a whole barramundi.

In Adelaide Magistrates Court on Tuesday, Quan was found not guilty of assault with the court ruling the charge had not been proved beyond reasonable doubt.

She was also found not guilty of acting unlawfully in not paying for a full fish.

Quan had argued that she had acted in self-defence during the incident in June last year.

During her trial, she said she repeatedly told waiter Fan Chi Kong that he had no right to stop her.

She told the court that she bit him after asking him a number of time to release her.

Mr Kong told the court that, as the only male waiter working at the time, his boss had asked him to follow Quan when she left the restaurant.

He said he grabbed her hand in an attempt to stop her getting away.

"When I grabbed her firmly she started biting me," he said through a translator.

Mr Kong said Quan continued to walk as she was biting him, prompting him to grab her purse before squatting to the ground.

"I continued grabbing her purse and waited until the police arrived," he said.

In his decision, magistrate Oliver Koehn said police had failed to prove that Quan had not acted in self-defence.

"I find that the prosecutor has not excluded the possibility that the defendant genuinely believed that the conduct in which she engaged was necessary and reasonable to defend herself from what she perceived to be an assault by Mr Kong," Mr Koehn said in his published reasons.

The magistrate said it was clear that Quan was manhandled by Mr Kong when he grabbed her and would not let go.

"She was entitled to defend herself from that action," he said.

"As to whether or not her conduct was reasonably proportionate, I bear in mind that the law does not require that the force used by a person to defend themselves must not exceed that used against them.

"When he (Mr Kong) did let her go as a result of the biting, the defendant ceased biting."

Quan declined to comment outside court.

Australian Associated Press