Emergency services were called to Charles Street in Liverpool just after 12.30pm on Tuesday following reports of a shooting.
NSW Ambulance said a 32-year-old man was treated for a single gunshot wound to his lower left leg before being taken to hospital. His injuries are not believed to be life-threatening.
A crime scene has been established and a large police presence remains on the street, about 30 kilometres south-west of Sydney's CBD.
Police have urged anyone with CCTV or dashcam footage, or any information about the shooting, to contact Crime Stoppers on 1800 333 000.