UPDATED 2.31pm: Christchurch schools are warning students to stick together, after a man sexually assaulted a 16-year-old schoolgirl in North Hagley Park.

Pauline Duthie is principal of Christchurch Girls' High, which is across the road from the park.

"We have put this in to our notices immediately, we will be sending a little alert home to parents, we'll be telling all of our girls to walk home with a friend and not unaccompanied."

Pauline Duthie said the victim is not a student at her school.

Police were alerted by hospital staff at 9pm after the girl reported the incident to them. She said it happened sometime tween 3.30pm and 8.25pm. 

The alleged offender is described as a very tall male part Maori, medium build with tattoos on both arms.  He was wearing black pants and a black hoodie.

Police want to hear from anyone who may have seen or spoken to the girl or the alleged offender at Hagley Park of in the vicinity of Christchurch Hospital.

“The investigation has progressed throughout the morning and Police have identified all relevant parties involved and are no longer trying to identify anybody else in connection to this,” said police in a statement

“Police believe this is an isolated incident and there is no going risk to the public.”