She was unconscious when taken to Rotorua Hospital on July 22, and never came out of her coma.
Among the allegations against the accused is that they placed Nia in a tumble dryer and turned it on, and spun her on a clothesline until she fell off. The dryer is one of 29 exhibits the Crown plans to present.
A long list of charges against the five defendants was read out at the start of the proceedings, and the Curtis brothers began by pleading not guilty to Nia's murder.
The courtroom was packed to standing room only with 91 potential jurors, 14 media representatives and 11 lawyers - as well as the five accused, flanked by security guards, in the dock.
It was impossible for reporters on the press bench to see the dock because of the number of people in the court, but Nia's mother, Lisa Kuka, Michael Curtis's partner Oriwa Kemp, and Nia's cousin Michael Pearson could be heard denying charges of manslaughter.
The Crown alleges that Kemp, 18, and Pearson, 20, were party to the unlawful acts of Wiremu and Michael Curtis, aged 19 and 22 respectively.
Kuka, 35, who was in a relationship with Wiremu, faces two manslaughter charges - one for allegedly failing to provide medical treatment for Nia, and the other for failing as a parent to take reasonable steps to protect her from violence.
Collectively, the four other defendants faced three charges of cruelty to a child and 20 charges of assaulting Nia and two other children when the trial began, but Wiremu Curtis and Kemp pleaded guilty to five charges between them.
Curtis admitted performing wrestling moves on the other two children, who cannot be named for legal reasons, and Kemp admitted throwing shoes at them and Nia.
Some of the remaining charges against the four are representative charges, which means the Crown believes the abuse outlined occurred on more than one occasion.
The allegations include that Kemp kicked Nia in the face, Michael Curtis lifted her up to the ceiling and dropped her, and he, his brother and Pearson performed wrestling moves on her.
Jury selection for the trial took more than an hour after Justice Potter warned those in the pool that they should make it known if they did not believe they could be impartial following significant publicity on the case.
Thirteen jurors were stood aside after approaching the judge, and once the final group of eight men and four women was empanelled, she ordered them not to read or watch media reports of the trial.
Just before the lunch break William Curtis, father of the Curtis brothers, entered the public gallery. He faces separate charges in relation to earlier alleged abuse of Nia, but a trial date for him has yet to be set.
