Homicide detectives say they have made a breakthrough in the investigation into the death of a man outside a Melbourne nightclub six years ago.

Police have released CCTV images from the Queensbridge Hotel at Southbank, showing a man who they believe witnessed a fatal assault.

Shannon McCormack, 22, died in hospital in 2007, a week after he was hit in the head while trying to break up a fight between his friends and another group of men.

Despite an extensive investigation and a $100,000 reward the case remains unsolved.

Police spokeswoman Jessica Foulds says police are asking anyone who knows the man shown in the security footage to contact them.

"This footage shows a man that is possibly in his 20s who was in the area at the time of the assault and police are looking to identify this man," she said.

The man police want to speak to is described as being of a slim to medium to build with blonde-tipped brown hair and was wearing a purple t-shirt.

The victim's father, Bill Mr McCormack, says investigators moved as quickly as they could to release the footage to the public.

"The reason the footage has only come to light is there were hundreds of hours of footage that people had to go through and a lot of the people were wearing similar clothes," he told ABC local radio.

"It was very hard to distinguish who it was out of hundreds of thousands of people in the footage they had."