A gunman who shot three brothers during a six-hour siege in Sydney's southwest is believed to have been in a "bikie business dispute over a sign" with his victims.

Police were called to a signage business on Heald Rd at Ingleburn where Wayne Williams, 33, was armed with a long-arm firearm and holding three men hostage.

Michael Bassal, 43, was killed when Williams opened fire. One of his brothers sustained serious injuries, while the other managed to escape with superficial wounds.

The brothers reportedly went to the factory to complain about a sign they had ordered, before the argument became heated and a gun was produced,The Daily Telegraph reported.

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Williams is believed to be a Finks bikie and his motive for the siege was reportedly related to a business dispute with the three men, the Sydney Morning Herald reported.

Williams was also found dead after police stormed the building just after 5pm. It is believed he shot himself.

Mr Bassal's Facebook page is filled with pictures of opulent motorbikes and cars and it appears he is known to his friends as Mick.

After police stormed the building, three male hostages were taken to the end of the exclusion zone to waiting ambulances. They were transported to hospital for observation.

The entire siege lasted six hours and 15 minutes.

As police moved in to retrieve the remaining hostages, witness Sunia Ram told Daily Mail Australia police moved into the building "very casually" and showed no "urgency in their movements".

Earlier in the day, police armed with large guns, helmets and and bulletproof vests concentrated their search for the shooter near a signage business called Inline National Signage and Property Services.

First responding officers on Monday morning found three people with gunshot wounds at the scene but one man died at the scene. Two others with gunshot wounds were in Liverpool Hospital where one underwent emergency surgery, while the other has superficial wounds to the lower part of his body.

A witness named Kevin, who works next door to Inline, said he heard the sound of gunshots.

"It was just pop, pop, pop, pop," he told Daily Mail Australia.

Kevin thought little of it until heavily armed police arrived a short time later and raced through his building and out the back roller door.

Another witness, who works 30 metres from the scene, said he was not aware of the incident until he walked outside.

"There was this body on the road covered by a white sheet, right there, and all these cops around the place," the man, who wished to remain anonymous, told Daily Mail Australia.

"Then I saw two cops from [Strike Force] Raptor stop this guy with mullet and tatts and search his car. They did the same to another bloke who was parked across the road."

- Daily Mail