Reward for NSW train child assault info

A $1000 reward is on offer for information about a man who indecently assaulted a 12-year-old girl on a Sydney train in November.

The incident happened at about 9.20pm on November 4, when the girl and her mother were about to leave the city bound train as it approached Wolli Creek station.

Police say a man in his mid-to-late 30's with a dark complexion and short, black hair reached under the girl's skirt and grabbed her buttocks.

The girl and her mother got off the train and immediately alerted police, who were unable to find him.

The man was later seen leaving Granville train station.

Police have offered the reward as part of a public appeal for information about the assault.