HMAS Sydney, named for the Australian city of Sydney, was one of three modified Leander-class light cruisers operated by the Royal Australian Navy (RAN). Ordered for the Royal Navy as HMS Phaeton, the cruiser was purchased by the Australian government and renamed prior to her 1934 launch.

During the early part of her operational history, Sydney helped enforce sanctions during the Abyssinian Crisis, and at the start of World War II was assigned to convoy escort and patrol duties in Australian waters. In May 1940, Sydney joined the British Mediterranean Fleet for an eight-month deployment, during which she sank two Italian warships, participated in multiple shore bombardments, and provided support to the Malta Convoys, while receiving minimal damage and no casualties. On her return to Australia in February 1941, Sydney resumed convoy escort and patrol duties in home waters.

On 19 November 1941, Sydney was involved in a mutually destructive engagement with the German auxiliary cruiser Kormoran, and was lost with all hands (645 aboard). The wrecks of both ships were lost until 2008; Sydney was found on 17 March, four days after her adversary. Sydney's defeat is commonly attributed to the proximity of the two ships during the engagement, and Kormoran's advantages of surprise and rapid, accurate fire. However, the cruiser's loss with all hands compared to the survival of most of the Germans has resulted in conspiracy theories alleging that the German commander used illegal ruses to lure Sydney into range, that a Japanese submarine was involved, and that the true events of the battle are concealed behind a wide-ranging cover-up, despite the lack of evidence for these allegations.

The ship was laid down by Swan Hunter & Wigham Richardson at Wallsend-on-Tyne, England, on 8 July 1933 for the Royal Navy as HMS Phaeton, named after the Greek mythological figure. However, in 1934, the Australian government was seeking a replacement for the light cruiser HMAS Brisbane, and negotiated to purchase Phaeton while she was still under construction.

The cruiser was renamed after the capital city of New South Wales, and was launched on 22 September 1934 by Ethel Bruce, the wife of Stanley Bruce, former prime minister of Australia and the serving Australian High Commissioner to the United Kingdom. Sydney was commissioned into the RAN on 24 September 1935, drawing her ship's company from Brisbane, which had been decommissioned earlier that day.

Following the announcement that Australia was purchasing a British-built cruiser, there was criticism, primarily from the Opposition of the day, stating that such a warship should be built using Australian resources and labour. Several reasons were given in reply for acquiring British-built cruisers instead of Australian-made: the ship was already close to completion, the pending threat of war meant that there was not enough time to train Australians in the necessary shipbuilding skills, and that of the two cruisers built in Australian shipyards, one (HMAS Adelaide) had taken seven years to complete.

Sydney was one of three Modified Leander-class light cruisers acquired by the RAN during the late 1930s. Although the first ship of the class to join the RAN, Sydney was the second ship to be laid down, although the first to be completed, in what was sometimes referred to as the Perth class: Perth and Hobart operated with the Royal Navy for a short period before they were purchased by Australia in 1938. Like most British cruisers, the Leanders were designed for long-range patrols, scouting, and trade protection duties.

Sydney's displacement ranged between 6,701 tons (light) and 8,940 tons (full load), with a standard displacement of 7,198 tons: improved fabrication and welding techniques made her 52 tons lighter than her sister ships. She had a length of 530 feet (160 m) between perpendiculars and 562 feet 4 inches (171.40 m) overall, a beam of 56 feet 8.5 inches (17.285 m), and a draught at standard displacement between 15 feet 3 inches (4.65 m) forward and 17 feet 3 inches (5.26 m) aft.