In late 1944 the Americans handed Bougainville over to Lieutenant General Stanley Savige's Australian II Corps and went to the Philippines. The Australians decided to finish what the Americans had contained. On 31 December 1944, Savige ordered Brigadier John Stevenson's 11th Brigade to begin clearing operations along the northwest coast. The goal was to push the Japanese into the narrow Bonis Peninsula and destroy them there. The vanguard was the 31st/51st Infantry Battalion under Lieutenant Colonel Joseph Kelly - roughly eight hundred men, militia troops, advancing along a coastal track through villages with names that meant nothing to anyone outside the island: Kuraio Mission, Amun, Sipaai, Rukussia. By 16 January 1945 the lead company had reached Rukussia without firing a shot. The next day that changed.

First contact came at a clearing a thousand yards north of Puto. A platoon patrol from D Company under Captain Thomas Titley attacked several huts, killed one Japanese soldier, and then ran into machine gun fire from entrenchments further on. The Australians captured the position, along with a field hospital and two Type 92 70mm battalion guns. They also captured documents identifying the defenders as the 81st Infantry Regiment. Meanwhile, on 20 January, forward patrols came upon Tsimba Ridge itself. Two hundred yards long, sixty feet high, running down to the Genga River on the west where a cliff fell to the river's southern bank. Its south-eastern tip, separated by a shallow saddle, the Australians named the Pimple. From its crest Nakamura's men could see everything. They had dug a continuous trench along the ridgeline, with weapons pits and pillboxes, all with clear fields of fire. It was the kind of position that could cost a battalion to take.

Kelly tried the obvious approaches first. Between 21 and 28 January, B Company attacked the ridge from the south, southeast and north. None worked. On 23 January he brought up the 2nd Mountain Battery for fire support. Three more attempts on 24 January failed. Then on 25 January C Company tried a different tack entirely: they crossed the crocodile-infested Genga River about six hundred yards from the ridge and established a bridgehead on the far bank, keeping the line of communication open by constant patrols. For six days the Japanese attacked that bridgehead with what survivor accounts recorded as bordering on suicidal determination. On 29 January they broke through one part of the Australian perimeter and rolled up the position all the way back to the battalion base at Puto before the counter-attack, organized by C Company's Captain Alwyn Shilton and supported by accurate artillery, stopped them. Shilton's counter on 1 February cost the Japanese more than thirty killed or wounded.

Kelly spent the first week of February preparing. A mountain gun was manhandled through the jungle to a position a hundred and fifty yards from the Pimple, where it could fire directly onto the Japanese trenches. Aircraft from No. 5 Squadron RAAF flew reconnaissance. To mask the sounds of digging, Australian mortars kept up regular fire, and Vickers machine guns swept the ridgeline to keep the defenders pinned. On 6 February at 09:00, after a day-long barrage and airstrikes from Australian Wirraways and New Zealand Corsairs, three platoons of B Company under Captain Millett Harris attacked from three directions. Ten Platoon captured its objective by 09:25. Twelve Platoon took heavy fire and stopped. By 11:30 Eleven Platoon had reached the high ground on the northwest, completing the encirclement. The Japanese pulled back to the reverse slope. That night their artillery hammered the Australian positions around the Pimple, and at dawn on 7 February they launched a banzai attack that was repulsed. On 9 February, after a final airstrike, the last defenders withdrew. The attack on 6 February alone had cost the Australians nine killed and twenty wounded.

When the Australians walked the ridge they counted the cost on both sides. They had captured four field guns, six anti-tank guns, nine machine guns and eighty-six rifles. Japanese dead were estimated at sixty-six in the Tsimba fighting, one hundred and forty-eight across the wider Genga River operations that followed. Australian losses in the Tsimba area itself came to twelve killed and twenty wounded - a number that the Australians and one newspaper called the bloodiest battle of the campaign. Private Colin Jorgensen received the Military Medal for charging a Japanese weapon pit during the final assault. After the war, captured Japanese documents told the other side of the story. Nakamura had been prepared to hold. He had the numbers. What broke him was not Australian firepower but the quiet grinding failures of supply - ammunition running short, food running shorter, tropical diseases climbing. The 31st/51st Infantry Battalion was awarded the battle honour Tsimba Ridge. Captain Clyde Downs, who led A Company in the flanking patrols of January, would die leading his men at the failed Porton Plantation landing that June.