The 15-year-old girl was walking her dog along the beach at Broulee just after 5.10pm when police say she spotted a man "committing an act of indecency" on a sand dune near a pathway.

She started to run away and was followed by the man, who stood up and tried to grab her shoulder but missed, police said.

When the girl left the beach, she told a woman nearby what happened and the woman called police.

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Police want to speak to a man described as being in his early 40s, unshaven and bald, around 170cm tall, with tanned skin and hairy arms.

Anyone with information is urged to call Crime Stoppers on 1800 333 000.