In 1909 the Admiralty proposed the creation of a Pacific fleet consisting of three fleet units to be operated by Imperial forces in the Pacific region. Each fleet unit would be built around a battle-cruiser, supported by several light cruisers. One of these fleet units would comprise the Royal Australian Navy. With Australian Government agreement to this concept, work was commenced on the ships. The light cruiser HMAS Sydney was built in the United Kingdom and commissioned in June 1913. On 4 October 1913 she entered Sydney Harbour with other ships of the fleet unit.
Following the outbreak of war in August 1914, Sydney was engaged in operations against Germany's Pacific colonies and destroyed a radio station in the Pelew Islands. Returning to Australian waters, Sydney joined the escort of the first troop convoy from Australia to the Middle East, which left Albany on 1 November 1914.
On 9 November, Sydney was detailed to leave the convoy to investigate reports of an unknown vessel off the Cocos (Keeling) Islands. This vessel turned out to be the German cruiser SMS Emden. In the resulting engagement, Sydney was hit early by the long-ranging 10.5 cm guns of Emden, which resulted in four dead and twelve wounded. However, the superior firepower of the Australian cruiser's broadside soon told and Emden was left "beached and done for" on North Keeling Island.
Sydney briefly rejoined the convoy at Colombo before proceeding to the Atlantic Ocean, via the Mediterranean Sea. In December 1914, Sydney was deployed to the Caribbean and based at Bermuda. For the next 18 months, she carried out patrol duties between the islands of the West Indies and along the east coast of North America, in concert with HMAS Melbourne. Both ships were also detailed for occasional special patrols along the north coast of South America.
In September 1916, Sydney and Melbourne were transferred to the 2nd Light Cruiser Squadron for duties with the Royal Navy's Grand Fleet in the North Sea. In company with HMS Southampton and Dublin, they undertook patrol, escort and screening duties. While on patrol on 4 May 1917, Sydney and the other vessels accompanying her were attacked by the German zeppelin "L43". The "L43" dropped ten to twelve bombs on the Sydney, none of which hit the ship, which responded with a heavy barrage of anti-aircraft fire. The "L43" escaped undamaged.
Later in 1917, Sydney was fitted with an aircraft launching platform and equipped with a Sopwith Ships Pup fighter. On 1 June 1918, while patrolling in the Heligoland Bight, Sydney and Melbourne launched their aircraft to engage two German reconnaissance aircraft. Sydney's fighter destroyed one of the enemy aircraft.
Sydney was present at the surrender of the German High Seas Fleet on 21 November 1918. She returned to Australia on 19 July 1919. After the war, Sydney carried out routine fleet duties until paid off in 1928. She was broken up the next year.