A detective close to the investigation said bikie gangs were their "main suspect", but they were yet to establish a motive for the November 13 attack that caused an estimated $200,000 in damage to foreshore bar and restaurant Republica.

"We're certainly aware of bikies causing problems at the venue in the past and we know that police have attended on at least one occasion to discuss the matter," the detective said.

Three offenders wearing hooded jumpers and scarves over their faces were seen throwing petrol bombs into the Jacka Boulevard restaurant in the early hours of the morning before they fled up Fitzroy Street.

It is believed that several LP gas bottles were also opened in the restaurant, but failed to ignite.

Police confirmed at the time that the offenders broke into the building during the attack, using a number of items inside to help light the fire.

The incident was caught on security cameras, but police were unable to identify the offenders, according to a source close to the investigation.

The restaurant is believed to have instituted a "no colours" policy and recently refused entry to former Bandidos sergeant-at-arms Toby Mitchell.

A Victoria Police spokeswoman said the investigation into the suspicious fire remains "active and ongoing".

"Detectives continue to follow up several lines of inquiry however at this early stage, have not found any direct links to OMCG activity," the spokeswoman said.

The attack on Republica comes after a member of the Mongols was arrested in a car park off Fitzroy Street in August for allegedly pulling a gun on security staff at popular nightclub Secret Garden.

Bikies have also been linked to the firebombing of a tattoo shop in South Melbourne in July shortly before it was due to open, with members of gangs operating business in close proximity to the City Road shop.