Police were first called to the property about 11.50pm on Saturday following reports of an assault at a 21st birthday party.

They were told a 16-year-old Lithgow youth had been taken to Bathurst Base Hospital for emergency surgery after being hit in the face with a glass object.

Police yesterday said the youth was later transferred to a Sydney hospital and there were fears he could lose sight in one eye.

Police from Bathurst, Lithgow detectives and the forensic services group returned to the home about 4am yesterday and executed a crime scene warrant.

A number of occupants of the home and party-goers were asleep when police arrival but as they secured the crime scene police allege they saw the 21-year-old woman try to discard a clear plastic bag containing what is believed to be cannabis.

Forensic investigators examined the scene and found a large amount of blood at the entrance to the residence and a blood trail which led to a laundry.

Police seized a number of items, including 425 grams of what is believed to be cannabis. The 21-year-old woman will be issued a court attendance notice for deemed supply of a prohibited drug, due to the large quantity.

Police were yesterday continuing their inquiries.