In late May, the Japanese high command determined that they would attempt to force the Australians off Lababia Ridge, and began making preparations for an assault, rotating their forces in the area, with the exhausted 102nd Infantry Regiment, which had been fighting the Australians since the Battle of Wau, being replaced by two battalions of the 66th Infantry Regiment, under the command of Colonel Katsutoshi Araki. Supplies and ammunition were brought up towards the position along the steep slope from the Komiatum Ridge, and on 20 June, Japanese patrols from the 66th, with guides from the 102nd, began probing the Australian position on Lababia Ridge, which was held by about 80 men. Japanese sappers defused the booby traps that the Australians had created out of piano wire and hand grenades, but being unfamiliar with the mechanisms of the Australian grenades, left them in place, rather than using them against the defenders. Sporadic firing occurred throughout the day, but no major attack developed. The following day, 21 June, Australian patrols noticed signs of Japanese activity along the track towards the observation post that had been established further east along the ridge. Around 07:30 the telephone line to the post was cut and shortly afterwards, Japanese troops approaching the main Australian position began setting off booby traps that had been set up as early warning devices. Around 14:00 the Japanese launched an assault from the north and north-east of the Australian position, supported by mortars. This was repelled, but followed up by another attack in the early afternoon, including a bayonet charge. Over the course of the afternoon, a total three attacks were made to the sound of bugle calls. While intended to frighten the Australian soldiers, these actually served to warn the defenders of their approach.
Attacker: Japanese (Lead and encircling elements of the 66th Infantry Regiment)
Defender: Australian ('D' Company, 2/6th Infantry Battalion and Smith's Platoon, 'D' Company)