A 14-year-old girl fought off an attacker while walking with her younger sister near Nelson on Sunday night.
The girl was assaulted near the Stoke Library and public toilets around 10.30pm.
She managed to fight off the man by kneeing him and pushing him away.
Police say the girl and her 12-year-old sister were left shaken by the incident.
The man was wearing a blue t-shirt with black on the shoulder, black shorts and white sneakers at the time off the attack.
He is described as Caucasian, roughly 175cm tall, of medium build with very short brown or blonde hair. It is believed he is between 40 to 50 years old.
Police would like to hear from anyone who was in the area on Sunday night and saw the man.
Anyone with information should call Nelson Police on (03) 546 3840 or Crimestoppers on 0800 555 111.