Photo: Victoria Police

Police have been told the 39-year-old woman was walking through a crowded dance floor at a bar on Acland Street about 1.30am.

An unknown man grabbed her from behind on the dance floor and sexually assaulted her.

He was last seen leaving the venue with another man, walking south-east along Acland Street about 2.20am.

The first man is believed to be aged 30 to 50-years-old with black hair, olive complexion and a medium build.

He was wearing a white shirt, black jeans and brown shoes

The second man is believed to be aged 30 to 40-years-old with a slim build.

He was wearing black shoes and a blue shirt with a white collar.

Investigators have released CCTV and images of the men they believe can assist them with their enquiries.

Anyone who has information about the incident or the man’s identity is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential crime report at www.crimestoppersvic.com.au.