In hindsight, Sydney Harbour's defences were both inadequate and largely ineffective against the Japanese midget submarine attack. Outside the harbour, there were six indicator loops on the ocean floor that stretched from the northern suburbs of Curl Curl to Dover Heights in the south. The loops, capable of detecting metal, recorded any movement in or out of the harbour. On the night of 31 May, four of the loops were out of service.
There were more indicator loops inside the harbour. Inner loop 11 stretched from North Head to South Head and Inner loop 12 stretched from Green Point, near South Head, across the inner harbour to Middle Head and Obelisk Bay. This indicator loop was laid in shallower water so it recorded all types of shipping, large and small.
Finally, there was an incomplete anti-torpedo boom net across the harbour. The completed section was anchored to the harbour bed, and stretched from the southern shore of Green Point, near Watson’s Bay, across to Georges Head near Clifton Gardens.
The net had two openings: one on the deeper channel on the western side of the harbour, which larger shipping used, and the other on the shallower, more difficult to navigate, eastern side of the harbour.
Construction had commenced in January 1942, but the floating boom gates which were to open and close the channels, were not in place. This left two gaps of 275 metres on the eastern side and 293 metres on the western side.
On the harbour’s cliffs and headlands, there were gun batteries, anti-aircraft batteries and searchlights.