Detectives from the Special Crime Investigation Section are investigating a sexual assault that occurred at Hayborough on New Year's Eve.

At around 10.30pm, Friday 31 December, a 16-year-old teenage girl became separated from friends while celebrating New Year's Eve on Dump Beach.

The girl was pushed to the ground and sexually assaulted by a dark haired man not known to her.

There were about 200 people at the beach at the time and it is believed a woman may have witnessed the assault as she spoke with the victim immediately afterwards. At this time investigators have not been able to identify the woman and are asking her to come forward.

Anyone with information about this incident is urged to call Crime Stoppers on 1800 333 000 or online at www.crimestopperssa.com.au