The 'Attack on the GP.55 Convoy' was the Japanese submarine attack on an Allied convoy on passage through the waters of eastern Australia from Sydney in New South Wales to Brisbane in Queensland (15/20 June 1943).

The convoy comprised 10 cargo vessels (seven British, two US and one Dutch) and three US tank landing ships escorted by five Australian corvettes, and was attacked by the Japanese submarine I-174 on 16 June, losing the 5,550-ton US Army transport ship Portmar and suffering severe damage to the US Navy’s LST-469. Two of the corvettes counterattacked I-174, but damaged the Japanese boat only lightly.

The Australian military undertook an intense search for I-174 in the days after the attack in the mistaken belief that the boat had been significantly damaged. This search was unsuccessful, and at the same time served to highlight the unsatisfactory communications between the Royal Australian Navy and Royal Australian Air Force. However, another Japanese submarine passing through the area may have been sunk by RAAF aircraft. Because of Japan’s deteriorating operational and strategic situations, I-174 was the last Imperial Japanese navy submarine to operate off the eastern coast of Australia.

During 1942 and 1943, Japanese submarines periodically operated in the waters surrounding Australia. A force of midget submarines raided Sydney Harbour on the night of 31 May/1 June 1942, and attacks on merchant shipping off the eastern coast began several days later. The attacks continued until August 1942, when the Japanese submarine force was redeployed.The 'Kaidai' class submarine I-174 completed a 24-day patrol off Australia in July and August 1942, but did not attack any ships. In response to the Japanese attacks, the Australian naval authorities ordered that all ships with a displacement of more than 1,200 tons and a maximum speed of less than 12 kt must travel in escorted convoys from 8 June. These convoys were successful in minimising losses, and no escorted ships were sunk off Australia during 1942.

Japanese submarines resumed operations in Australian waters during January 1943. I-21 made a successful patrol in January and February, in the course of which it sank five ships, and two other submarines operated off Sydney and Brisbane in March. A force of five boats from the Imperial Japanese navy’s 3^rd Submarine Squadron subsequently attacked shipping along the full length of the Australian eastern coast during April and May in an attempt to disrupt the Allied supply line to New Guinea. This marked the peak of the Japanese submarine offensive against Australia, and nine ships were sunk off the eastern coast within a month. By contrast with the situation in 1942, several successful attacks were made against ships travelling in convoys.

The large number of Japanese attacks in 1943 put great strain on the Allied forces responsible for protecting shipping off eastern Australia. The Australian naval authorities were forced in April to reduce the number of convoys that sailed so their escort could be increased to at least four warships. The RAAF also greatly increased the number of aircraft allocated to escort convoys and ships sailing independently. More warships and aircraft fitted for anti-submarine warfare became available in May, but were still not adequate to counter the Japanese attacks.

The sinking of the Australian hospital ship Centaur on 14 May 1943 with heavy loss of life led the Advisory War Council to seek information from the Australian navy and air force on the actions being taken to protect shipping. While the navy acknowledged that the 'Bathurst' class corvettes constituting most of its escort force were too slow, it argued that the losses suffered by escorted convoys were no worse than those in other parts of the world. Despite the navy’s assurances, Australia’s anti-submarine forces were constrained by a shortage of training opportunities and the poor co-ordination between the Australian navy and air force with each other and with the US Navy. The Australian military had also not kept pace with improvements to British and US anti-submarine doctrine by fully implementing the tactics that had proved successful in other theatres.

I-174 departed the major Japanese naval base at Truk in the Caroline islands group on 16 May 1943 under the command of Lieutenant Nobukiyo Nanbu, and arrived off Sandy Cape, Queensland, on 27 May. I-174 was the only Japanese boat operating off Australia at the time, as all other available submarines were deployed to counter Allied advances in the Solomon islands group. The boat delivered an unsuccessful torpedo attack on the 3,303-ton US Point San Pedro on 1 June, exchanged gunfire with US Army transport vessel Edward Chambers three days later, and on 5 June was driven off by the escorts of the PG.33 convoy. On 7 June, I-174 fired four torpedoes at the US 'Liberty' ship John Bartram, but all these missed their target. The submarine sighted another convoy on 13 June, but this was too far away to be attacked. During this period, I-174 was repeatedly attacked by Allied aircraft and warships, but sustained no damage.

The GP.55 convoy assembled in the middle of June 1943 as one of at least 69 convoys which made the passage from Sydney to Brisbane in 1943. As noted above, the convoy comprised ten cargo vessels (Carlisle, Charles F. Meyer, Colorado, Cycle, Giang Ann, John Bartram, Mangola, Matthew Flinders, Portmar and Time) and three US Navy tank landing ships, and was escorted by the 'Bathurst' corvettes Warrnambool (carrying the convoy’s senior officer), Bundaberg, Cootamundra, Deloraine and Kalgoorlie. After departing Sydney at 08.45 on 15 June, the convoy settled into five columns, with three ships in each of the two central columns and two in the single outer columns. The escorts surrounded the convoy, with four sailing of it and Deloraine astern. RAAF Avro Anson twin-engined coastal and Bristol Beaufort twin-engined attack bombers also patrolled over the convoy at all times. The US Army transport Portmar, which had been badly damaged in the 'Bombing of Darwin' on 19 February 1942, had difficulty maintaining her position in the convoy and at times straggled behind the other ships.

I-174 sighted the GP.55 convoy about 40 miles (65 km) to the east of Smoky Cape at 16.37 on 16 June, immediately began preparations to attack and easily penetrated the escort screen. At this time Portmar was trying to return to her station and was passing to port of LST-469. This made the ships an ideal target for Nanbu as they overlapped in the view of his periscope. Accordingly, Nanbu fired two torpedoes at 17.20. One torpedo struck the tank landing ship near her stern two minutes later, resulting in severe damage, the loss of 26 lives and the wounding of 17 men. Portmar spotted the second torpedo and attempted to evade it, but was also struck on her starboard side. The transport’s cargo of petrol and ammunition quickly caught fire and the ship sank within 10 minutes. One of her crew and one passenger were killed, and 71 survivors, including four injured persons, were rescued by Deloraine. Despite losing steerage way, LST-469 remained afloat and was taken in tow by the corvette. I-174's attack on the GP.55 convoy was probably the most successful made by a Japanese submarine off Australia.

While Deloraine attended to the torpedoed ships, the other four escorts attempted to locate the Japanese submarine. I-174 had not been detected as it approached the convoy, and after the attack the Australian corvettes reversed their courses to conduct a sonar sweep of the area from where the submarine was assumed to have attacked. This was in accordance with tactics that had recently been adopted by the Australian navy after they proved successful in the 'Battle of the Atlantic'.
Warrnambool detected the submarine 23 minutes after the attack, and in company with Kalgoorlie subjected I-174 to four depth charge attacks over two hours until losing contact. An Anson aeroplane of No. 71 Squadron was escorting the convoy at the time the two ships were torpedoed, but was short of fuel and had to return to base shortly after the attack. While the corvettes believed that that they had sunk I-174, the boat was only lightly damaged and withdrew to the east. The Australian failure to sink the submarine was due to a lack of training and practice, and to the availability of too small a number of ships to create an adequate search scheme.

After breaking off their attack, Warrnambool rejoined the convoy while Kalgoorlie aided Deloraine in protecting the damaged tank landing ship. Deteriorating weather broke the tow-line between Deloraine and LST-469, and instead the corvette proceeded to nearby Coffs Harbour with Portmar's survivors and the tank landing ship’s wounded personnel. The naval tug Reserve was despatched from Brisbane on 16 June to recover the tank landing ship and towed her to Sydney, which they reached on 20 June. LST-469 had been carrying troops and supplies for the 'Chronicle' amphibious landing on Woodlark and Kiriwina islands on 30 June, and this operation was hindered by her unavailability.

After the attack, the Australian chief of naval staff, Vice Admiral Sir Guy Royle, judged that the submarine had been damaged and ordered 'special measures' be taken to search for it. The RAAF began air searches of a 100-sq mile (260-km²) box to the south-east of Coffs Harbour on the night of 16/17 June, while Deloraine, Kalgoorlie and the recently arrived Australian destroyer Vendetta patrolled the area. On 17 June, Anson aircraft were assigned to patrol the submarine’s most likely escape routes in the hope that this would force it to remain submerged during the day and surface at night. Radar-equipped Beaufort aircraft relieved the Anson machines after dusk and continued the search.

Early in the morning of 18 June, two Beaufort warplanes of No. 32 Squadron attacked with bombs and gunfire what they took to be a submarine. Deloraine was less than 6 miles (9.7 km) from the area of the RAAF attack, but did not acknowledge repeated attempts by the aircraft to communicate with her. The Beaufort aircraft reported that they had damaged the submarine and intensive efforts were made on 19 June to locate and sink her, with 12 Anson machines continuously sweeping the area while six Vultee Vengeance single-engined dive-bombers were held in readiness nearby. No further sightings were made and it was assumed that the submarine had been damaged by a No. 32 Squadron aeroplane but escaped. A naval historian has written that the sighting made on 18 June was probably mistaken as I-174 was at least 60 miles (97 km) to the east of the location in which the RAAF attack took place, and the incident was not recorded in the submarine’s war diary. However, other historians believe that the Beaufort aircraft may have sunk I-178, which was patrolling off the eastern coast of Australia at the time and disappeared some time after 17 June 1943. I-174 was ordered to depart Australian waters on 20 June, and returned to Truk on 1 July. The boat made no contact with any Allied ships or aircraft after attacking the GP.55 convoy.

The apparent failure of the Australian navy and air force to co-operate in order complete the destruction of a damaged submarine led Royle to order an inquiry. This found that a breakdown in communication between the two services was the main cause of the failure, with both the aircraft and the naval signal room in Sydney making procedural errors. Rear Admiral G. C. Muirhead-Gould, the naval officer in command of the Sydney region, also noted that communications between the navy and the air force were unsatisfactory at higher levels, and that the communication procedures to be used in anti-submarine operations were neither well known nor understood. In response, he suggested that the navy establish the position of Commander Escort Vessels Group to organise and command escorts and co-ordinate their tactics.

The attack on the GP.55 convoy was the last attack made by a Japanese submarine on the eastern coast of Australia. Two submarines were dispatched to operate off Australia in July 1943, but were diverted to the Solomon islands group shortly before reaching the eastern coast. After this, the Japanese submarines were fully occupied responding to Allied offensives and transporting supplies to isolated garrisons. As the threat to shipping declined, the navy ceased running convoys in waters to the south of Newcastle on 7 December 1943 and the convoys between Sydney and Brisbane were terminated on 10 February 1944.