A woman who was allegedly indecently assaulted by a flasher at Sandringham beach, in Melbourne's south-east, has managed to take a photo of her attacker.

The woman, aged in her 40s, was walking her dog along the foreshore path earlier this month when she was approached by a man wearing a towel.

Police said he exposed himself to the woman before indecently assaulting her.

Detective acting inspector Brad Daly said she managed to take a photo of her attacker before he was disturbed by a passersby and fled.

"I'm sure she was very stressed but she was cool enough to take a photo of him," he said.

It is believed he may have left the area in a silver hatchback.

Police have released the photo taken by the woman as part of an appeal for help.

Anyone with information can contact Crime Stoppers on 1800 333 000.