The 'Raid on Port Gregory' was undertaken by a Japanese submarine as part of attempts to divert Allied attention away from the 'Ke' (i) evacuation of the Japanese forces from Guadalcanal in the Solomon islands group (28 January 1943).
None of the 10 shells fired at Port Gregory caused any damage, and the attack was unnoticed by the Allied naval authorities until a radio signal sent by the commander of the Japanese submarine was intercepted and decoded a week later.

During the Pacific War Japanese submarines occasionally operated off Western Australia. Early in 1943, the major Japanese headquarters directed their forces to make small attacks on Allied positions, in an attempt to divert attention away from the planned evacuation of Guadalcanal. In the middle of January, Vice Admiral Takasu Shiro, commander of the Japanese South-West Area Fleet, ordered Lieutenant Commander Tatenosuke Tosu’s I-165 to attack Allied shipping off north-western Australia. As part of the fleet’s contribution to diversionary operations, I-165 was also instructed to shell an Australian coastal town. It is likely that the busy port of Geraldton was the original target for the bombardment.

I-165 departed Soerabaja in the occupied Netherlands East Indies on 21 January 1943. Another Japanese submarine, I-166, was ordered to conduct a similar mission against the Cocos islands.

During the evening of 27 January, I-165 arrived just to the north of Geraldton. While preparing for the attack, the crew of I-165 spotted three aircraft and a surface vessel which Tosu believed to be a destroyer. The Japanese captain therefore decided to postpone the bombardment, and made a temporary withdrawal to a point farther to the north. While running on the surface, the submarine passed within 2 miles (3.2 km) of what was identified as a destroyer, without being noticed.

During the following night, at just after 00.00, I-165 surfaced 7,000 yards (6400 m) off Port Gregory, and at a range of 7,655 yards (7000 m) her crew fired about 10 rounds from the submarine’s 100-mm (3.93-in) deck gun at a derelict crayfish cannery, which they had misidentified as an ammunition factory. None of the shells caused any damage. Tosu did not attempt to observe the results of the bombardment, and resumed patrolling off the coast of Western Australia.

Two Australian coastwatchers stationed near Port Gregory noticed the gunfire, but neither observed the fall of any shells. The Allied naval authorities learned of the attack only after the submarine’s battle report radio signal was intercepted and decoded a week later. I-165 returned to Soerabaja on 16 February without having sunk any Allied ships during her patrol.

The attack on Port Gregory was one of three submarine shellings on Australian towns and cities, the other two being the attacks on Newcastle and Sydney in June 1942.