Police are appealing for help after a man approached two girls on a Sydney train and allegedly performed a sex act.

Investigators released an image tonight of a man they want to speak to about the January 12 incident.

The two 15-year-olds were travelling on the top carriage of a T4 line train from Bondi Junction to Central Station when they were approached about 2.45pm, police were told.

Police said the man “performed a sexual act in the presence of the girls” and left the train at Central.

Tonight they released an image of a man “who might be able to assist with inquiries”.

They described him as being about 30 years old with a large build and olive skin, wearing a black and white hat, basketball-style jacket and purple shorts.

The jacket was open exposing his stomach, police said.

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