3 September 1939
Before the outbreak of war in 1939, Australia had based its security on the ‘Main Fleet Strategy’. It was the idea that, in the event of war with Japan, the British would send a fleet to Singapore. This would provide Australia with the defence it needed to maintain coastal trade and sea communications. War with Germany, and then Italy, caused Britain to reconsider this strategy.
Without the guarantee of a British fleet, Australia was vulnerable to Japanese submarines and minelaying ships. Adequate defences against these were needed to maintain the safety of ports and shipping focal points. This was not only for Australia’s sea communication, but also its coastal trade. It was expected to increase in a war with Japan, as roads and rails would be at capacity.
The government began a local shipbuilding drive. It would ensure that Australia had anti-mine and anti-submarine ships to protect coastal trade. The result was the Bathurst-class corvettes, of which 60 were built. Australia also started producing asdic (sonar) devices and depth chargers. Supplies from Britain were needed to service needs in the Atlantic.
The coastal and international trade routes needed to be considered as one. They were both interconnected and complementary. This meant that the interruption of one would have implications for the other.
The presence of Japanese submarines off the east coast of Australia in early-to-mid 1942 was typified by the midget submarine attack in Sydney Harbour. Thus, it forced the introduction of coastal convoys.
As with international convoys, naval ships would escort groups of domestic merchant ships between destinations to ensure their safety. They were introduced for traffic on Melbourne–Newcastle and Sydney–Brisbane journeys. Trans-Tasman sailings from Sydney and Brisbane were escorted in convoy out to 200 nautical miles from the coast.
By November 1943, it was recommended that coastal convoys south of Brisbane stop. The following January, it was suggested that the Sydney–Brisbane and Brisbane–Gladstone convoys should end. This didn’t mean anti-submarine measures stopped completely, just that convoy escort along the east coast did. As the Japanese were pushed north, so was the convoy system.
As Japanese submarine attacks off the east coast of Australia slowed, U-boat operations took their place. This was as the German Navy’s commander in chief, Grand Admiral Karl Dönitz, attempted to divert Allied units from important German interests. The suspected presence of U-boats in the Indian and Pacific oceans tied down Australian units. It resulted in alleged sightings, but they were not enough to re-constitute the coastal convoy system.