Sydney Harbour anti - submarine boom net The anti - submarine boom net was an anti - torpedo and submarine defence net that was in Sydney Harbour during World War II . It spanned the entire width of the harbour from Green Point , Watsons Bay to Georges Head , on the northern side of Sydney Harbour . The boom formed part of the Sydney Harbour defences which also included artillery batteries and patrol boats . History . Construction of the boom commenced in January 1942 and the boom and gates were fully operational by August 1942 . For over three years , entry to Sydney Harbour was restricted by the boom net . All vessels including ships , ferries and fishing boats gained access to the harbour via one of three gates in the anti - torpedo and anti – submarine boom which spanned the harbour from Green Point to Georges Head . On the night of 31 May 1942 , three Japanese midget submarines attempted to enter the harbour in what became known as the Attack on Sydney Harbour . On the night of the attack the central section of the net was complete and support piles were in place to the west , but there were gaps of up to 400 metres ( 1 , 300 ft ) wide on either side of the boom net . Material shortages , not lack of interest , prevented the completion of the boom net prior to the attack . The first submarine , the M 27 , commanded by Lieutenant Jahai Chuma , was the first submarine to attempt entry . The M 27 followed another vessel into the harbour , but at some stage became entangled in the western end of the boom net ' s central section . Lieutenant Chuma tried to free his submarine by using a sawtooth metal cutter , which only entangled it further . By 8 . 30 pm he had attracted the attention of the watchman for the Maritime Service Board , James Cargill , who rowed across in a boat and discovered the Japanese submarine . Surprised , Cargill radioed naval headquarters . It took him two hours to convince the navy that there was a Japanese midget submarine caught in the boom net . At 10 : 27 pm Chuma , realising that he had been detected , detonated charges , destroying the submarine and killing himself and his crew in the process . The two other midget submarines penetrated the then only partially completed boom net , and attacked shipping for several hours before being destroyed . The boom net was removed after the end of the war in August 1945 , and by early 1946 , everything apart from the dolphins had been removed . The foundations of the winch house are the only remaining evidence of the anti - submarine boom net and they can still be seen on the shores of Green Point , along with other remaining evidence of military installations , such as underground fortifications and bunker complexes . Adjacent to the winch house stood a stone marker that had existed for 90 years prior to the boom net ' s construction . Erected in 1850 , this was used as a navigational leading mark for the Eastern Channel ; it was lined up with the back mark which still stands on the west side of Parsley Bay . Boom gates . The western gate was usually kept shut and was only opened for very large vessels that were damaged or under tow , which required the use of the deeper and straighter Western Channel . The western gate was opened by a tug boat . All other ships used the eastern gate , which was controlled by the boom gate vessel . The gate was opened by dragging it back to the hauling - back dolphin using winches housed on Green Point . The small craft gate was lowered to a depth of 5 metres ( 16 ft ) with a winch mounted on board HMAS " Kuramia " . Dimensions . The boom was long , with two gate openings for ships . The gate on the Western Channel was in length and was used for larger vessels . The gate on the eastern side of the channel ( Eastern Channel ) was long , and there was another gate for smaller boats located on the western side of the boom gate vessel ( tug boat ) , which was long . The vessel used to pull theses gates open was HMAS " Kuramia " , a converted Sydney Harbour ferry , that had been used for public transportation from 1914 . The central and western sections of the boom consisted of some 49 clusters of four piles with the net , which was made of wire and interlocking steel rings about in diameter , suspended between them . The eastern , Green Point , section of the net was suspended from buoys , as were the eastern and western gate nets .