The 24-year-old man was found with multiple gunshot wounds inside his silver sedan on Hertfordshire Street in the southwest Sydney suburb of Elizabeth Hills about 4pm on Monday.
"Despite the efforts of police and NSW Ambulance paramedics, the man died at the scene," NSW Police said in a statement on Tuesday.
The man is yet to be formally identified, but he has been named in media reports as Marvin Oraiha, a low-level associate of the Comanchero bikie gang.
Police Commissioner Karen Webb said it was still early in the investigation, but she acknowledged concerns the shooting could reignite Sydney's gangland wars.
"Of course, we don't know that it is gangland-related at the moment," she told Seven's Sunrise program.
"We will have investigators working around the clock on this to make sure that we can suppress any retaliation if that is a concern as well."
Police are investigating whether two cars found burning a short time later at Green Valley and Bonnyrigg Heights are linked to the crime.
Strike Force Hackenberg has been formed to investigate.
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