A 15-year-old boy was at a playground in a park on Highett Street in Richmond when he was approached by an unknown male around 7.30pm on March 25, according to police.
The man allegedly struck up a conversation with the boy then sexually assaulted him, before fleeing the scene.
Police have released a photo of a man they want to speak to about the incident.
The man is perceived to be Asian in appearance and aged between 25 and 35.
He was wearing dark-rimmed glasses, a dark-coloured t-shirt, light-coloured cargo shorts, dark-coloured sneakers and a silver coloured-watch.