It’s understood the woman was sitting on a bench at Terry Oval near Gosford, at around midnight on Wednesday, when she was approached by a man.
Officers have been told the man started a conversation with her, before sexually assaulting the 26-year-old.
The man is described as being aged in his 30s, of Caucasian appearance with a solid build and light brown or blonde hair.
He may also have facial injuries suffered during the incident.
He was last seen running from the oval towards Emma James Street and Headlam Parade.
Police are asking anyone with information, or who saw a man matching the description hanging around Terry Park to contact Crime Stoppers on 1800 333 000.