A teenage boy was knocked to the ground and sexually assaulted while listening to his iPod in Melbourne's north on Wednesday.

Detectives say the 17-year-old was at the Quarry Hills Park lookout in Epping on Wednesday when he was bashed unconscious and assaulted.

The South Morang boy told police he was listening to his iPod and looking at the view between 7.15pm and 7.30pm when a man struck him from behind, knocking him to the ground.

The man sexually assaulted the boy, who was dazed on the ground, and struck him on the jaw during the assault, causing him to lose consciousness.

A couple passing by found the victim unconscious and called an ambulance and police.

The victim's clothes were in disarray. Police said he was taken to hospital with cuts and grazes.

The offender is described as being aged between 25 and 30, of Asian appearance and chubby build with a flat nose, pale coloured skin and shaven legs. He had short brown hair which was noticeably longer in the front and was wearing a white t shirt, blue shorts with black pockets and white, slip-on shoes.

Epping sexual offences and child abuse investigation team detectives will release an image of the man on Friday.

Detectives also want to speak to a potential witness who was seen walking his dog in the area at the time of the offence.

The man is described as being aged in his 60s, about 170cms tall, solid build with sandy grey hair and was walking a golden brown Labrador or similar.

Police are appealing for anyone with information to phone Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au.