A gunman is on the run in Sydney's south-west after shooting a man in the leg and threatening two other people with a gun at a nearby home.

The shooting on a quiet suburban street in Greenacre on Friday morning happened metres from the gates of St John Vianney Catholic Primary School, which was put into lockdown for over an hour.

Witnesses say at least three shots rang out on Pandora St about 9.30am before they found Robert Chorbadjian sprawled on the driveway of a church.

Police believe the gunman shot Mr Chorbadjian before threatening two other people in a nearby home and then speeding from the area in a car.