A large pool of human blood has been found inside a cubby house in the playground of a school in southern Sydney and police want to know who it belongs to.

The blood was found by staff at Riverwood Public School on January 31. Police took samples for forensic testing and results have now confirmed the blood is human.

Due to the amount discovered, police are concerned that someone may have been seriously injured during an incident in the playground.

Police are talking to the school but believe the incident occurred during the summer holidays and that there is no link to staff or students.

DNA testing is now being carried out to try to find the potential victim after checks with local hospitals failed to link any people seeking treatment to the scene at the school.

Police want anyone who may have seen people in the playground between January 13 to 29, or who has any information about the incident, to come forward.