Firefighters were called to Kurri Kurri Hospital on Friday morning after a small flaming object was thrown through a window into one of the buildings at the Lang Street facility.
The object burned out before firefighters needed to take action, though plenty of smoke was seen at the site about 7.30am.
The Newcastle Herald understands the burning material was not thrown into a ward room or space containing patients.
Hunter New England Health was aware of the incident and police have been informed.
Police said the small fire caused "minimal damage to the room".
Hunter Valley Police District detectives are making inquiries.
Anyone with information that could help with the investigation is being asked to contact Crime Stoppers.
