That's the difficult question currently facing a judge in the ACT Supreme Court, where prosecutors are arguing a mum who forcibly removed her eight-year-old daughter from her grandmother's home should be found guilty of assault.

The case revolves around a series of events that took place in Canberra in December 2012.

The young girl had been living with her grandmother for some time, although there were no court orders or barriers preventing her mother from having custody.

A family feud over money was splitting the family, particularly the girl's mother and grandmother.

A fight between the pair on the night of the offence ended "acrimoniously", and the girl's mother turned up at her home in the early hours of the morning.

She went into daughter's room, woke her up, "forcibly removed" her, and took her back to her house. The child was not found to have been dragged, although there was some force involved.

Police were called, and the mother was charged with common assault.

The case came before Magistrate Peter Dingwall in the ACT Magistrates Court.

He found that the mother had been acting within her rights as a parent to use limited force to compel the child go with her.

"In my view, what occurred was an not unreasonable application of force by the mother to give affect to her parental power to insist the child go with her," Mr Dingwall found.

That finding meant there was what's called a "lawful justification" for the assault, and the woman was found not guilty.

On Friday, Director of Public Prosecutions Jon White appealed that decision in the ACT Supreme Court.

He argued the magistrate should not have looked at the case by focussing on the right of the parent. It should have instead been looked at through the right of the child to autonomy of her person.

The girl, he argued, was clearly taken against her will in an effort to punish the grandmother in the context of a family feud.

She had been living with her grandmother for some time and did not want to go with her mother.

Mr White argued she simply would not have gone unless the force was applied.

"That child has just as much right to autonomy of her person as any other person does," he argued.

"It becomes less complex the older they get, there's no doubt about that.

"But the matter needed to be looked at through the autonomy of the child, and it wasn't."

But lawyer Paul Edmonds argued the magistrate had clearly understood the law and had made no error in his decision.

Mr Edmonds argued that the mother, as guardian of the child, legally had "limited powers of physical custody".

"Whether it was mistaken or not, she was taking her daughter home because that was what she thought she wanted," Mr Edmonds said.

"She was taking her daughter to her home without excessive force.

"We say, your honour, that that is really the start and end of the inquiry in this case."

Acting Justice Stephen Walmsley said the case raised "significant issues" and reserved his decision, saying he would need further time to consider it.

He is likely to hand down his decision in March.