Denzel Martinez became startled when a train staff member caught him lying down on a seat and masturbating while on a stationary train at Kiama Railway Station about 3.50am on Tuesday, April 16.

The 26-year-old Hurstville man quickly removed his hand, however the witness soon after caught Martinez repeatedly pleasuring himself and reported the incident to police.

Martinez was still masturbating when the train stopped at Oak Flats where officers were waiting for him.

He quickly pulled his pants up and was asked to leave the train, initially refusing to tell them his name. Police checks revealed he was on bail for the same offending.

Martinez dialled into Wollongong Local Court from the police holding cells that afternoon and pleaded guilty to two counts of wilful and obscene exposure in a public place.

Asked why he was travelling so late, Martinez replied: "That's when I go see my friends."

"This behaviour in public is completely unacceptable ... this is frightening to people," Magistrate Gabriel Fleming said before imposing a 12-month community corrections order with supervision.

Martinez was also ordered to seek mental health treatment and was banned from travelling on any trains between 10pm and 6am, unless in the company of a family member.