Man gunned down in front of son outside Sydney gym

A man has been executed in front of his 12-year-old son during a drive-by shooting in southwest Sydney.

Police said the 40-year-old-man was outside the Elite Fight Force mixed martial arts gym in Sefton just after 6.30am on Thursday when the killers struck.

The man, named in media reports as Taha Sabbagh, was going to the gym to train when the assassins pulled up in a Mazda car and shot him multiple times in the chest as his 12-year-old sat in the passenger seat of their vehicle.

Detective Superintendent Danny Doherty described the murder an "egregious, violent act".

"These people are callous and brazen," he told reporters on Thursday.

"They have no regard for human life knowing there was a 12-year-old boy in the car and still carrying out the execution in front of him."

The victim's son was physically unharmed but Det Supt Doherty said the event had taken a mental toll.

"His father's been shot in front of him. It's very traumatising for him and his family," he said.

Police found a burnt-out Mazda 3 about a kilometre away in neighbouring Birrong and they believe the vehicle might help identify the assailants.

While Mr Sabbagh had links to the criminal underworld, Det Supt Doherty said police were keeping an open mind about the motives behind his murder.

"At this stage, there is no evidence of it being connected to any previous conflicts that have plagued the city in the last 18 months. We're treating it as an isolated incident," he said.

Det Supt Doherty asked anyone with dashcam footage or other information to come forward and assist with the investigation.