"She then went up to her and flicked her in the side of the head, causing her to feel dizzy," Mr Walker said.
"She then twisted her arm up behind her back and the girl heard a click in her shoulder and neck."
The girl burst into tears and had to be taken out of class.
The next day the girl's mother noticed her daughter's arm was bruised and took her to hospital, where the fracture was discovered.
In a recorded police interview, the girl told Constable Lucy Kennedy she had been doing the actions correctly.
She said Sister Leva was "angry as" and had red cheeks when she approached her. After Sister Leva twisted her wrist behind her back, her arm went "dazy and all sleepy".
"She [asked] why am I crying and that I'm crying in an old lady's way." All the other helpers in the room started laughing, the girl said.
Giving evidence by closed circuit TV from another court room, the 9-year-old said she cried until her mother came to pick her up.
She said her arm still hurt sometimes - more than a year later.
Under cross-examination from defence lawyer Andrew Comeskey, the girl agreed she did not see Sister Leva flick her in the head but later said the nun was in front of her when it happened.
Mr Comeskey suggested to the girl she had got the actions wrong and Sister Leva was trying to correct her.
The girl answered: "I'm not sure".
In his opening address, Mr Walker reminded the jury to ignore feelings of prejudice and sympathy.
"The issue is heightened in this trial because the fact is that the accused is a nun. That may provoke sympathetic emotions for you. I'd be surprised if it didn't."
He said Sister Leva's occupation was "completely irrelevant".
Wearing a cross on her lapel, Sister Leva used her cardigan to shield her face from photographers as she left court with supporters.
The trial, before Judge Heemi Taumaunu, is set down for four days.
