The boom net spanned the entire width of Port Jackson and a boom net winch house was located on Liangs Point, Watsons Bay. On the night of 31 May 1942, three Japanese midget submarines attempted to enter Sydney Harbour in what became known as the Attack on Sydney Harbour. One of the Japanese midget submarines became entangled in the boom net and after unsuccessful attempts to free the submarine, the crew detonated charges within the sub, killing themselves and destroying their sub in the process.
During this period Nielsen Park was used as an anti-aircraft base. The base had its own interim wooden barracks, searchlights and anti-aircraft guns.
In Gap Park, a gun emplacement for a small calibre gun, which was never installed was also intended for use during World War II
