A mobile phone image has helped police lay charges over an alleged indecent assault in an underground car park in Manuka on Monday.

Police allege that an 18-year-old man followed a woman to her car in the underground car park of Coles Manuka about 10.45pm.

The man is accused of performing an act of indecency while the woman was sitting in her car.

Police were able to quickly find the 18-year-old after a witness provided a mobile phone image of the man she had taken earlier that night, when police say he was acting suspiciously.

He was charged with act of indecency without consent after being taken to the ACT Watch House.

He was bailed and will appear in court on January 29.