Police in Sydney are investigating a second possible mistaken identity killing at the hands of organised crime in less than 48 hours.

About 15:15 local time on Wednesday, emergency services rushed to a home in Merrylands West after reports a man in his 40s had been targeted in a drive-by shooting. He died at the scene.

A day earlier, about 03:40 on Tuesday, a university student in his 20s with no criminal ties, was shot during a home invasion in Glen Alpine in the city's southwest. He also died at the scene.

Police say the Tuesday shooting appears to be an "innocent life" taken by organised crime while Wednesday's fatal attack may have targeted the "wrong person".

Police believe the man had arrived at the Merrylands West home just moments before he was shot in the doorway.

Asked if investigators were treating the case as a suspected mistaken identity, police said: "Tragically, that is a possibility that the wrong person has been shot by these offenders".

"The victim has no links that we are aware of to organised crime, however he has associated with people that reside at this address," Det Supt Simon Glasser said on Wednesday.

The residents of the home were "very well known to police" with links to organised crime, but the man who was shot was "quite likely" a business associate, Glasser said.

Earlier, New South Wales Police Commissioner Mal Lanyon had described Tuesday's killing of Marco Tapia, a university student who was only known to police over a traffic offence, as "completely unacceptable".

"The fact that what appears to be an innocent life was taken [on Tuesday] is something that no community should accept and no community should live in fear of violence," Lanyon said.

"I don't want any innocent member of the community to be impacted by organised crime. We will not tolerate it."

Shortly after Tuesday's fatal shooting, a known organised crime figure posted in a message on social media: "popped the wrong person, didn't even get the right house".

Earlier this year, a Sydney grandfather was allegedly kidnapped and murdered in what police believe was a case of mistaken identity.

Chris Baghsarian, 85, was taken from his home in a pre-dawn raid and his body was found two weeks later near a golf course. Several people have been arrested and charged in the matter.

And last year a Sydney plumber was shot dead in front of his family home in what police said was also a case of mistaken identity.