The iconic landmark had been cordoned off following unspecified threats stemming from "information on social media," police said. People were cleared from the harbor before police declared the area safe.
Metal barriers were erected Thursday and onlookers kept about 150 meters (500 feet) away from the building, while police officers and security personnel guarded the area in a 90-minute lockdown.
"Following information on social media, police conducted an operation in the vicinity of the Opera House and Manly as a precautionary measure," a police statement said.
No further details were provided although Sydney's Daily Telegraph newspaper said the scare revolved around a threat received about an object inside the Opera House.
An Opera House spokeswoman refused comment.
Australia is on heightened alert for attacks by grassroots militants. Authorities claim they have thwarted a number of potential attacks, although there have been several "lone wolf" assaults.
jar/rc (Reuters, AFP, dpa)