Simon Partridge was wearing a G-string with the face of a donkey on the front when he pulled down his pants "for a few seconds" in front of passengers on a train at Bulli station.

Partridge, 18, of Park Rd, Bulli, told the court he had worn the underwear after accepting a dare from friends and pulled down his pants to show a friend who he said was travelling on the train.

Police were at the station and saw the incident.

When they approached Partridge, he fled and later resisted arrest.

At the time of the incident on March 28, Partridge was on a good behaviour bond for a separate matter.

The court heard that since recently ending his swimming career, during which he competed at a national level and for Corrimal Swimming Club, Partridge had "let his hair down".

Magistrate Ian Guy criticised Partridge for lasting only four days after entering the bond before he again broke the law.

Partridge was fined $500 for wilful and obscene exposure in a public place and $700 for resisting arrest.