Sydney is situated on a submergent coastline, where rising ocean levels have flooded deep rias, creating a diverse coastal landscape. The city is defined by its geographical boundaries, with the Tasman Sea to the east, the Blue Mountains to the west, the Hawkesbury River to the north, and the Woronora Plateau to the south.

The topography of Sydney spans two distinct geographic regions. To the south and west of the Harbour lies the Cumberland Plain, a relatively flat area that was among the first to be developed for urban use. In contrast, the Hornsby Plateau is located to the north of the city, characterized by steep valleys that shape the region's landscape. The construction of the Sydney Harbour Bridge was a pivotal moment that enabled the northern suburbs to experience significant population growth.

Sydney's coastline features seventy surf beaches, with Bondi Beach being the most iconic and well-known among them. This picturesque setting attracts both locals and tourists alike, contributing to Sydney's reputation as a vibrant coastal city.

The Nepean River wraps around the western edge of Sydney and transitions into the Hawkesbury River before reaching Broken Bay. Most of the city's water storages are located along the tributaries of the Nepean River, playing a crucial role in the region's water supply. The Parramatta River, primarily an industrial waterway, drains a significant area of Sydney's western suburbs into Port Jackson. Meanwhile, the southern parts of the city are serviced by the Georges River and the Cooks River, both of which flow into Botany Bay.

Defining the boundaries of Sydney can be complex, as there is no single accepted definition. The Australian Statistical Geography Standard outlines that Greater Sydney encompasses an area of 12,369 km² (or 4,776 sq mi), incorporating local government areas such as Central Coast in the north, Hawkesbury in the north-west, Blue Mountains in the west, Sutherland Shire in the south, and Wollondilly in the south-west.

The City of Sydney, the local government area, covers approximately 26 square kilometers, stretching from Garden Island in the east to Bicentennial Park in the west, and extending south to the suburbs of Alexandria and Rosebery. This intricate interplay of topography and urban planning has been pivotal in shaping Sydney’s development and its vibrant character.