Police swarmed near Sydney's Stadium Australia, in the city's west, but have been unable to find the man who was carrying the gun.
Officers were called to Uhrig Road, Lidcombe, about 7.20am on Tuesday over reports a man armed with a handgun threatened a woman in a multi-level car park.
The man was not known to the woman and fled in a white Subaru SUV using fake number plates, police said.
The woman was left shaken but not physically injured.
Heavily armed police, some carrying semi-automatic weapons and wearing camouflage, set up a perimeter around the area, a block of housing adjacent to the 80,000-seat Olympic stadium.
The incident follows a fatal shooting inside a unit in nearby Shale Street on Monday.
Two men have since been charged with the murder of the 38-year-old man, in what police described as a gangland-style attack.
The victim was rushed to hospital about 4am on Monday but died a short time later. A second man was found at the scene with non-life-threatening injuries.
Officers arrested a 21-year-old man on Monday afternoon after a car crashed into a parked vehicle, NSW Police said.
Police found a firearm in the car and later charged the man with murder and wounding a person with intent to cause grievous bodily harm.
Detectives then raided a home on Spurway Street in Ermington, near Lidcombe, about 8pm and arrested a 22-year-old man.
He was also charged with murder.
Both men have been refused bail and are expected to face Burwood Local Court on Tuesday.
Police earlier said they believed the shooting was carried out by offenders with links to organised crime.
"This is a targeted attack. This is not a random attack," Superintendent Robert Toynton told reporters on Monday before the arrests.
He had described the gunmen as heavy-set and dressed in black clothes.
Members of the public have been urged to come forward with any information relevant to either of the incidents.
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