Police are looking for seven men over the murder of a Hells Angel bikie in Sydney's west.

Zelco Mitrovic, 45, was gunned down inside his transport company's office at Wetherill Park on Tuesday afternoon.

Police say seven men argued with Mitrovic before three of them followed him to his upstairs office.

They argued again before one of them pulled out a pistol and shot him several times.

Another employee in the room was shot in the arm.

Strike Force Lambourn has been set up to investigate, led by the Homicide and Gangs Squads.

Acting State Crime Commander Peter Cotter says it is unlikely another bikie gang is behind the murder, but the motive still isn't clear.

"At this stage we think it's more related to a personal side of business or a personal side of his life," he said.

Police are examining CCTV footage from the building and are continuing to interview witnesses.