Police are investigating after a man died in hospital following a shooting at an apartment in Sydney's south-west.

Officers arrived at an apartment block in Warwick Farm, on the corner of George and Lachlan streets, about 5.20pm on Sunday following reports that a man had suffered a gunshot wound.

Police were told two unkown men, one armed with a firearm, knocked on the front door of the 46-year-old's apartment.

They then entered the unit and shot him in the back before leaving, police say.

The victim was treated at the scene before being taken by ambulance under a police escort to Liverpool Hospital. He died that night in hospital, police said.

Officers from Liverpool City Police Area Command with assistance from the Dog Unit conducted an extensive search of the area, but could not locate the men.

Several people who witnessed the shooting are assisting police with their inquiries.

"I just saw plenty of police cars going down the street, plenty of ambulances going down the street and it was just crazy," one witness told Nine News.

"Police blocked off the road straight away."

A crime scene is being maintained and forensic crews remained inside the apartment on Monday morning.

The area around the unit block is still cordoned off to the public as police continue their investigation. They believe it was a targeted incident.