The victim was found seriously injured near the playground at the Richmond Village Green off Stanmore Rd on Wednesday, February 5, and the teen was arrested and initially charged on February 28.
A 16-year-old appeared in the Christchurch Youth Court last week charged with wounding with intent to cause grievous bodily harm.
He is also facing a charge of assault in relation to a second victim and one of theft after a separate offence in the same area two days before.
On Friday, police filed further charges against him, relating to serious violent sexual offending.
He appeared in the Christchurch Youth Court and has been remanded in custody.
The accused boy was scheduled to reappear in the Christchurch Youth Court in early April.
"The victim is slowly recovering from a significant head injury and is still receiving treatment as part of her long road to recovery," Detective Senior Sergeant Karen Simmons said.
"We are continuing to provide her with support and update her about developments in this case.
"We would like to thank the members of the public who have come forward with information and allowed us to get to this point."
