The 19-year-old is in hospital with four gunshot wounds to his stomach and arm following the shooting in Lansdowne Street, Merrylands, about 10.50pm on Wednesday.

Another man, aged 62, was shot in the side of the chest when he came out of his house to investigate an argument he heard on the street. He underwent surgery on Thursday morning and is listed in a critical and unstable condition.

A 27-year-old man was arrested at a home on Lansdowne Street on Thursday morning and has been charged with armed robbery causing grievous bodily harm and shooting with intent to cause grievous bodily harm.

Police on Thursday morning said the shootings were believed to have been the result of a bungled armed robbery, and the two injured men were "innocent victims".

Police said the teenager had driven to Merrylands with two friends, aged 17 and 18, to buy a mobile phone that had been advertised for sale on a social networking site.

When they reached Lansdowne Street, they were flagged down by two men. Soon after, two more men approached the car.

One of the men, who was armed with a gun, got into the back seat of the vehicle and demanded money and other personal items from the teenagers, police said.

An argument broke out and shots were fired inside the car, injuring the 19-year-old, police said.

A NSW Police spokesman said the teenagers drove to a nearby petrol station on the corner of Woodville and Merrylands roads in Merrylands, about one kilometre from the shooting scene, to raise the alarm.

The injured teenager was treated by paramedics before being taken to Westmead Hospital, where he was in a serious but stable condition on Thursday morning.

Detectives set up crime scenes at the petrol station and on Lansdowne Street, and are looking for four men.

One has a thin build, a small amount of facial hair and tattoos across his body, including one on his face.

Another has a short, thin build, a beard and tattoos.

The other two men have not been described.

Police said officers executed a search warrant at a house on Lansdowne Street at 6am on Thursday and arrested a 27-year-old man.

He was taken to Parramatta police station where he was charged.

He was refused bail and was due to appear in Parramatta Local Court on Thursday afternoon

Detectives from the Rosehill Local Area Command and officers attached to Operation Talon, formed in August 2013 to combat public place shootings in Sydney, are investigating.

They have urged anyone with information to contact Crime Stoppers on 1800 333 000, or Operation Talon on 1800 802 980.