0727 GMT July 17, 2018
About 1.5 million people packed the city’s foreshore to watch the pyrotechnics light up the sky above the historic bridge and the iconic opera house, the first major celebrations worldwide after New Zealand.
Tens of thousands turned out earlier in New Zealand's largest city Auckland for the annual New Year's Eve street party, marked by a major fireworks display from the Sky Tower.
As the New Year dawned in this southern hemisphere nation, fireworks boomed and crackled above city centers and harbors.
Celebrations moved to Asia, the Middle East, Africa, Europe and finally the Americas, with dazzling light shows bidding farewell to 2017.
Stricter security has been a key focus amid fears that crowds could be targets for vehicle and other terror attacks.
In Australia, the stronger police presence included some officers carrying semi-automatic rifles in Sydney and bollards used as barriers against vehicles.
Other cities are also on alert following deadly vehicle assaults over the past two years in Barcelona, Nice and London.
New York's Times Square celebrations were set to go ahead despite the Arctic chill gripping much of the central and northeastern United States and Canada.
In a message, Iranian President Hassan Rouhani congratulated the New Year to Iran’s Christian community.
Looking ahead to 2018
Daesh’s defeat in Iraq and Syria was one of the key stories of 2017, although the terrorists remain a threat and numerous attacks around the world were claimed by them or Al-Qaeda-linked groups.
Donald Trump stole the news spotlight after making his debut as US President in January 2017, with "America first" policies and a bombastic personal style that has shaken up international diplomacy.
The former reality television star is likely to continue dominating headlines in 2018, with escalating tensions over North Korea among a host of global challenges.
Other political and diplomatic earthquakes set to rumble into 2018 include the crisis in the Middle East between Saudi Arabia and its allies against Qatar, and the humanitarian disaster in Yemen.
In Europe, further talks on Brexit will help shape the region's future trade relationship while Russia is set to host the football World Cup amid frictions with the West.
AFP and AP contributed to this story