A 28-year-old woman who responded to a call for help in Mosman Park was hit over the head and knocked to the ground before a man tried to remove her pants.
Police are calling for help with their investigation into the assault, which happened about 7.20pm on Wednesday.
The victim was walking along Victoria Street when she heard a man shouting for help.
She walked in the direction of the calls and saw a man standing at the corner of Buckland Avenue and Victoria Street.
When she approached the man he hit her over the head with an object, possibly a soft drink bottle, grabbed her and punched her, knocking her to the ground.
He then grabbed at the waistband of her pants before he stopped and ran off along Buckland Avenue, towards Wellington Street.
The victim was treated for cuts to her head, lips and arms.
The man is 32 to 38 years old and about 178cm tall with a fair complexion, stocky build and short light brown or blonde hair.
He has acne scarring on his cheeks and was wearing black pants and a light blue T-shirt.
He was not wearing shoes.
Anyone who can identify the man or who has information on the incident is urged to contact Crime Stoppers on 1800 333 000.