Sydney Harbour defences Sydney Harbour was protected by coastal batteries and other fixed defences from the early 19th century until the 1960s . These defences were constructed to protect the Australian city of Sydney from attack by enemy warship s and submarine s . Pre - Federation defences . Sydney ' s earliest fortifications were built around Sydney Cove to protect the new colony of Sydney from attacks from non - British ships or convict uprisings . The first gun emplacement was built in 1801 on Middle Head ( north of Obelisk Bay ) during the Napoleonic wars . More permanent fortifications were built during the first half of the 19th century in response to feared foreign attacks and new threats to the British empire . In 1790 the Dawes Point Battery was meant to be the first line of defence against an attack by the Spanish Empire , Napoleon ’ s French troops in 1810 , and the Russian Pacific Fleet in the 1850s ( during the Crimean War ) . This was part of a line of defences that were concentrated on protecting the inner Harbour , these also appear to have been given a low priority . Construction of Fort Denison began in 1841 , though the fort wasn ' t completed until 1857 . Sydney ' s defences were expanded following the withdrawal ( Cardwell reforms ) of British soldiers from Australia during the 1870s . This prompted the construction of four sets of fortifications on Georges Heights and around other parts of the harbour , the Middle Head Fortifications , the Georges Head Battery , the Lower Georges Heights Commanding Position and a smaller fort located on Bradleys Head were constructed during this period and upgraded in the 1880s on the advise of British experts . They were designed to attack enemy ships as they attempted entry into Sydney Harbour . Some of these forts were , at the time , built of an out dated design . A Submarine Miners ’ Depot was constructed at Chowder Bay ( Georges Head ) in the 1880s . In 1888 the site was modified for the latest in harbour defences . The site was a strategic position and considered the best place to observe and fire mines which were laid underwater . Minefields were laid across the main shipping channels of Port Jackson from 1876 to 1922 and a base was built at Chowder Bay for the submarine miners ( Clifton Gardens ) . From Georges Head , miners watched for ships entering the harbour . Their job was to explode the mine closest to an approaching enemy ship . Each underwater mine was attached to an electric cable that ran up the cliff to the firing post . The work of the submarine miner was secretive , technical and dangerous . During a demonstration in 1891 , a crowd of several thousand watched as a terrible accident killed four miners and injured another eight . The Submarine Mining Corps was dismantled in 1922 . Sydney ' s defences were expanded over the last decades of the 19th century and eventually incorporated a fort on Bare Island to defend the approaches to Botany Bay . World War II . Sydney ' s defences were not expanded from their pre - Federation condition until the 1920s . During the 1920s a number of well - constructed fortifications were built to protect Sydney . Sydney ' s defences were further expanded following the outbreak of World War II and were considerably augmented once Japan entered the war . While Sydney ' s anti - submarine defences were not yet complete at the time of the attack on Sydney Harbour on the night of 31 May 1942 , they were successful in sinking two of the three Japanese midget submarine s which attempted to enter the Harbour . The end of coastal defence . Following the end of the Japanese threat to Australia Sydney ' s defences were reduced from 1944 . The last fortifications were not decommissioned until the 1960s , however . While Sydney is no longer protected by fixed defences , a high proportion of the Australian Defence Force is based within the greater Sydney region and could be called on to protect the city during an emergency . During the 2000 Sydney Olympics security for the Harbour was provided by Special Forces , Navy Clearance Divers and warships . Current major ADF facilities in Sydney include :