Which country does Sydney belong to?

Sydney belongs to Australia.

Sydney, the capital of New South Wales, Australia's largest city and port. On both sides of Port Jackson, where the Tasman Sea extends 20 kilometers into the mainland, there is a 1,150-meter-long iron bridge spanning the harbor. It covers an area of ??1,735 square kilometers. The population is 4,198,543 people (2003) and the population density is 345.7 people per square kilometer. Inner Sydney is the most densely populated place in Australia, with 4,023 people per square kilometer.