Most notably, Sydney is host to the UNSC's headquarters, a massive base called HIGHCOM Facility Bravo-6. The base, nicknamed "the Hive", stretches three kilometers underneath the surface. It is where members of the UNSC Security Council discuss matters concerning the safety of the UNSC, Earth, and its colonies.
Sydney is also the location of the Sydney Synthetic Intellect Institute, or "Double S.I.". SSII worked on the creation of AI constructs, theorizing new approaches and calculating the details necessary for the creation of the AI.
Sydney is located on Australia's south-east coast and is the state capital of New South Wales; it was also the site of the first European colony in Australia, established in 1788 at Sydney Cove by Arthur Phillip, leader of the First Fleet from Britain.
The city is built around Port Jackson, which includes Sydney Harbour, hence the nickname, "the Harbour City". It is Australia's largest financial center and the economic capital.
Sydney is a major international tourist destination, often referred to as the international gateway of Australia. The metropolitan area is surrounded by national parks, and contains many bays, rivers and inlets. By the 26th century, Sydney's distinctive landmark, the Sydney Opera House, still stands in the Sydney Harbour.
During the Battle of Earth, Sydney was devastated by Covenant forces due to its importance to the UNSC war effort. By January 26th, 2553 - Australia Day - only one flagpole remained standing in Sydney Harbor. However, Facility Bravo-6 survived the battle undamaged.
 List of appearances
- Halo: First Strike (First appearance)
- Halo: Ghosts of Onyx (Mentioned only)
- Halo: Glasslands
- Halo: Combat Evolved Anniversary (Terminals only)
- Halo: The Thursday War