The victim was found seriously injured at the Richmond Village Green on Stanmore Rd on February 5 and the teen was arrested and initially charged on February 28.
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."
