Shane Bowden, 48, was shot 21 times with an automatic pistol as he returned home from the gym in October 2020.

Those on "the fringe" of the alleged premeditated murder will be offered indemnity if their information results in a conviction, Detective Superintendent Brendan Smith said on Thursday.

"We're confident that it's the Mongols outlaw motorcycle gang actively involved in this murder," he said.

The killers may have been motivated by a "personal vendetta" after Bowden shifted his allegiance from the Mongols to rival gang the Finks, police say.

He was also shot and wounded in Melbourne three months before his death.

"We all know they don't take kindly to members chopping and changing their gangs or their affiliations, and that could be one of the motives," Det Supt Smith said.

He was hopeful a quarter of a million dollars plus an opportunity for indemnity would help break the bikies' "code of silence".

"A murder carries life imprisonment, that's a long time to be sitting in jail particularly for a young person. The better part of your life is gone before you see the light of day," he said.

The pistol used in the attack was described as "for all intents and purposes" a machine gun.

Police believe two main offenders, who were part of a larger group, are responsible for the alleged murder.

Members of the public are also eligible for the reward if their information leads to a conviction.

"The opportunity for someone to get a reward or indemnity will hopefully encourage those people to come forward and give us what we need," Det Supt Smith said.

Australian Associated Press