Convoy GP55 was a convoy of Allied ships that travelled from Sydney to Brisbane in June 1943 during World War II. It comprised ten cargo ships, three Landing Ships, Tank and an escort of five corvettes. The Japanese submarine I-174 attacked the convoy on 16 June, sinking the United States Army transport ship Portmar and damaging USS LST-469. Two of the corvettes subsequently counter-attacked I-174, but only lightly damaged her.
The Australian military conducted an intensive search for I-174 in the days after the attack in the mistaken belief that she had been significantly damaged. This search was not successful and highlighted the unsatisfactory communications between the Royal Australian Navy (RAN) and Royal Australian Air Force (RAAF). Due to Japan’s deteriorating strategic situation, I-174 was the last Imperial Japanese Navy (IJN) submarine to operate off the Australian east coast.