Four girls, aged between 11 and 12, were wading into the water at Bonbeach about 1.40pm on November 26, when the man passed them, a police spokesman said.

The man was jogging south towards Patterson River, but reversed direction after he passed the girls in the water.

"As the man drew closer, the girls noticed he had no pants on and was exposing himself towards them," the spokesman said.

"They sought refuge at a nearby house belonging to a relative of one of the girls, while the man fled north along the beach."

The jogger has been described as middle-aged, of solid build and was wearing a blue cap and grey singlet.

Moorabbin police's sexual offences and child abuse team are appealing for anyone with information to call Crime Stoppers on 1800 333 000.