Eighteen-year-old Salim Hamze and his father were discovered by police with gunshot wounds inside a ute at Guildford just before 9am on Wednesday.

The younger man died at the scene and 64-year-old Toufik Hamze was taken to Westmead Hospital in a critical condition, where he also died.

Police believe two offenders are responsible for the double murder but don't know what motivated it.

The ABC is reporting that a sports car pulled up outside the property before a man got out and opened fire on the pair.

The victims were "loosely related" to the Hamzy crime family, which has been in a violent turf war with with the rival Alameddine family, State Crime Command Director, Detective Chief Superintendent Darren Bennett, told reporters on Wednesday.

"We're worried about reprisals, we're worried about family feuds, and we're also worried about drug and kidnapping turf wars and they're going on in southwest Sydney all the time," Det Supt Bennett said.

The underworld feud most recently claimed the life of Bilal Hamze, who was ambushed and executed by gunmen in Sydney's CBD in June.

Salim Hamze was well known to police as someone who'd been under threat and was involved in criminal organisations, Det Supt Bennett said.

He was out on bail after being charged with firearms offences last month.

His father had no known involvement with any criminal activity.

Police don't yet know which of the pair was the target and where they sat in the large Hamzy crime group.

Det Supt Bennett said public place shootings were down this year.

"(But) when incidents like this take place, it seems like we're back to square one and we're investigating another double murder."

Specialist forensic officers are examining the crime scene on Osgood Street.

Raptor Squad officers, who specialise in stopping gangland violence and bikie gangs, have started proactive intelligence-based targeting of known members and associated gangs, NSW Police say.

Australian Associated Press