NT Police are calling for witnesses following a sexual assault at Berry Springs Nature Reserve this afternoon.

An eight-year-old girl was at the park with her parents and was swimming in the middle pool around 4pm when she was assaulted by a man unknown to her.

She was able to get away and alerted her father to the assault.

Witnesses say the offender was last seen headed toward the car park area.

Police are seeking to speak with a man at the scene who may be able to assist with their investigation. He is described as a Caucasian male approximately 40 years of age. He had short grey hair and a short grey goatee. He was wearing black and white board shorts.

Anyone who may have seen this man or know of his whereabouts is asked to contact Police on 131 444 or Crime Stoppers on 1800 333 000.