Wollongong police are investigating after a woman was indecently assaulted on a train travelling north of Thirroul last month.
The 23-year-old woman boarded a northbound train at Wollongong about 10.50am on Sunday, November 19.
Police say a man sat beside her somewhere between Thirroul and Waterfall stations, and began touching her thigh area.
"This has obviously caused her some distress," Wollongong Police Inspector Don Faulds said.
"She's had to get up and leave and she's reported that the following day to Wollongong Police."
Police are now looking for a man they believe could assist with their inquiries.
According to CCTV footage he has a dark complexion, possibly of Indian appearance, with short black hair and wearing a blue chequered shirt, light blue jeans and sneakers.
"Any sort of incident like this on any sort of public transport in the community is concerning for police," Insp Faulds said.
"[The man involved] doesn't know who this girl is - he's a total stranger to her - but he thought he was able to touch [her] inappropriately. Obviously he needs to be spoken to and dealt with by police.
"We'd like to find out who the person [in the CCTV footage] is, so we can either eliminate him from our inquires, or find out his version of events."
Contact Wollongong Police on 4226 7899 or Crime Stoppers on 1800 333 000 to assist with the investigation.