SYDNEY - A man shot dead in Tuesday's gun atrocity in southwestern Sydney was associated with a crime gang, the New South Wales (NSW) Police said.

Emergency services were called to a home on Bardo Circuit at Revesby at about 12:30 am (local time) on Tuesday, following reports of a shooting, police said.

Two men were found inside the garage of the home suffering multiple gunshot wounds.

One of the men was found with fatal injuries, while the other was shot in the leg and stomach and remains in hospital in serious conditions.

NSW Deputy Police Commissioner Nick Kaldas said the dead man had not yet been formally identified, however he is believed to be Mahmoud Hamzy, the cousin of jailed Brothers 4 Life gang boss Bassam Hamzy.

"Both were well known to us and both men were affiliated with Brothers 4 Life," Kaldas told a media conference on Tuesday.

"Clearly these people are involved in a criminal group and clearly the motives logically would have to be something to do with the illegal activities they've involved themselves in."

Kaldas said the two were at least attacked by three people in what could have been an inside dispute among gang members.

"I know the group is called Brothers for Life but there is also a feeling amongst some police that they're not really brothers for life, they're lucky to be brothers for today," he said.

"They have as much conflict within the group as they do with outsiders."