The 'Battle of the Treasury Islands' was fought between Allied (New Zealand and US) and Japanese forces for the Treasury islands between New Georgia and Bougainville islands of the Solomon islands group (27 October/12 November 1943).

Most of the ground forces were provided by Major General H. E. Barrowclough’s New Zealand 3^rd Division.

The Allied invasion of this Japanese-occupied islands group was designed to secure Mono and Stirling islands so that former could be used for the construction of a radar station and the latter could be employed as a staging area for the forthcoming 'Cherryblossom' assault on Bougainville. The attack on the Treasury islands group would serve the Allies' longer-term 'Cartwheel' strategy of isolating Bougainville and Rabaul, the latter the main Japanese regional base area on New Britain island, and the elimination of the Japanese garrisons of the area.

As part of the Allied strategy to isolate Bougainville and Rabaul and eliminating the substantial number and strengths of the Japanese garrisons in the area, late in 1943 the Allies decided, within the context of the continuing Solomon islands campaign, to launch an attack on the Treasury islands group. The undertaking, which was to be conducted primarily by New Zealand ground forces supported by US forces, was codenamed 'Goodtime'. For this operation, Brigadier R. A. Row’s New Zealand 8^th Brigade Group, part of the New Zealand 3^rd Division, was assigned to the US III Amphibious Force, which assigned its Southern Force under Rear Admiral George H. Fort for the operation.

Comprising a pair of islands, Mono and Stirling, the Treasury islands group is located 300 miles (485 km) to the north-west of Guadalcanal island, 60 miles (97 km) from Vella Lavella, and just 18 miles (29 km) from the Shortland islands group. At the time of the battle, the islands offered the Allies further opportunities to bypass large groups of Japanese forces as they advanced through the Solomon islands group in the direction of the main Japanese base around Rabaul, whose reduction was a key part of the overarching Allied strategy developed as 'Cartwheel'.

The Treasury islands group’s larger land mass is Mono Island, to the north, while the considerably smaller Stirling island to the south encloses the excellent anchorage of Blanche Bay. Mono island is dominated by three old volcanoes reaching a maximum height of 1,053 ft (321 m), and its only beaches for amphibious landings were those near the village of Falamai on the southern coast near the eastern entrance to Blanche Bay. Almost completely undeveloped at the time the Pacific War began, Mono island possessed only a rudimentary road net supporting copra plantations, and had been occupied by the Japanese early in 1942.

The islands are endowed with a deep natural harbour in the form of Blanche Bay, which the Allies determined would be useful for supporting landing operations at Cape Torokina on Bougainville. As a result of its high features, Mono island offered the prospect of serving as a radar station to provide early warning for air and naval surface attacks during the Cape Torokina operation. The Allies also hoped that the landing would convince the Japanese that their next move would be on the Shortland islands group, off Bougainville’s southern tip, or Buin on the southern tip of Bougainville opposite the Shortland islands, rather than the real invasion area in the Cape Torokina and Empress Augusta Bay area on Bougainville’s western coast.

The Treasury islands had been scouted by PT-boats on 21/22 October, and again by a small party of US marine raiders, landed from the submarine Greenling, on 22/23 August. The scouts reported that the Japanese garrison numbered just 225 men.

The Allies launched 'Goodtime' at 06.06 hours on 27 October. Three echelons of high-speed transports (destroyer conversions) had been assembled for the operation. In addition, there were eight LCIs, two LSTs, eight LCMs, three LCTs and two APcs. As the small-scale reconnaissance operations were being undertaken, the assault force conducted rehearsals off Florida island off Guadalcanal. The ships carrying the assault force were escorted by the US destroyers Conway, Cony, Eaton, Pringle, Philip and Renshaw, and a novel feature of this landing was the use of two LCI gunboats, the first LCIs so modified in the Pacific theatre.

On 27 October, following a short naval and air bombardment, seven destroyer conversions arrived in the transport area to the west of Cummings Point on Stirling island and began to lower their small landing craft, which were assigned to land forces on either side of the harbour. Despite heavy rain, which reduced visibility, the US destroyers Philip and Pringle fired a heavy, but ultimately ineffective, pre-landing bombardment. At the same time, the radar-equipped Eaton provided fighter direction capability. After this, the 29^th and 36^th Battalions landed around Falamai, on the southern coast of Mono island, some 2 miles (3.2 km) away from Blanche Bay’s western entrance. Meanwhile, a detachment of the 34^th Battalion, landed on Stirling island, while another detachment of 200 men of the 34^th Battalion, supported by the destroyer conversion McKean, skirted around the western side of the island and landed to the north around Soanotalu, to provide security for the radar station that was to be installed there.

Some 3,795 men landed in the assault wave, with the remainder of the Allied force came ashore in four waves during the following 20 days, for an eventual total of 6,574 men. 'Goodtime' was the second combat operation undertaken by the New Zealanders in the Pacific theatre after the 'Dogeared' undertaking on Vella Lavella in the previous month. The New Zealand infantrymen were supported by US combat support and service support units including the 87^th Naval Construction Battalion, a signals unit, a naval base unit, and the 198^th Coastal Artillery Battalion to provide anti-aircraft fire support.[18]

The Japanese were caught by surprise and were unable to scramble aircraft to attack the assault craft until after the troops had landed. Subsequently, late on 27 October, a force of 25 Aichi D3A 'Val' single-engined dive-bombers attacked the US destroyers Cony and Philip, and in the ensuing air fight 12 Japanese aircraft were shot down by supporting Air Solomons fighters and naval gunfire, while Cony was hit aft twice, resulting in the death of eight of her crew and the wounding of 10 others. The destroyer was taken in tow and brought back to Tulagi for repairs.

On shore, meanwhile, the fighting continued. The Japanese resistance to the initial landing was light, and was quickly overcome with only a small number of casualties in the first wave of the assault. Over the course of several hours, the beach-head around Falamai was secured in the face of merely sporadic Japanese resistance, and then over the following days patrols were sent out to clear the island. Meanwhile, the force holding Soanotalu fought off several sharp attacks between 29 October and 2 November, including one attack by a company-sized element that resulted in about 40 Japanese being killed. On Stirling island, the New Zealanders had been virtually unopposed and after landing had settled down to a routine of patrolling and base development. There were a few minor Japanese raids, but the Japanese air effort was focused largely on responding to the 'Cherryblossom' landing around Cape Torokina, which began on 1 November.

The British flag was raised over the ruins of Falamai, the islands' main settlement, and civil administration was restored on 1 November. Mopping-up operations began and over the course of 11 days a number of minor engagements took place as patrols sought to flush out Japanese hiding out mainly in caves on the northern coast. These engagements resulted in further casualties on each sides, including several groups of Japanese killed in firefights with New Zealand patrols. On 12 November, the islands were declared clear of Japanese forces, although Japanese hold-outs were sighted in the jungle into January 1944.

In conjunction with the 'Blissful' raid on Choiseul island, the 'Battle of the Treasury Islands' served to divert the attention of Lieutenant General Haruyoshi Hyakutake’s 17^th Army, responsible for the defence of the Solomon islands group, from the next major Allied target in the Solomon islands campaign. The success of the operation also helped to improve the planning of subsequent landings in the Pacific theatre, in which the New Zealanders' next combat operation would be the 'Battle of the Green Islands' in the 'Squarepeg' landings early in 1944. Casualties during the 'Battle of the Treasury Islands' amounted to 226 for the Allies, in the form of 40 New Zealanders killed and 145 wounded, and 12 Americans killed and 29 wounded. The Japanese lost 223 men killed and eight taken prisoner.

'Seabee' construction personnel of Company A of the 87^th Naval Construction Battalion, along with a 25-man detachment from its Headquarters Company, landed on 27 October. One 'Seabee' raised the blade on his bulldozer to use it as a shield, and attacked a Japanese machine gun nest. The 'Seabees' built 21 miles (34 km) of roads, and established a PT-boat base on Stirling island. The initial detachment was supplemented by the rest of the 87^th Naval Construction Battalion on 28 November, and the unit then started work on the construction of an airstrip 5,600 ft (1705 m) long and 200 ft (61 m) wide, along with taxiways, hardstands and an aviation fuel farm with five 1,000-barrel storage tanks. In December, the job was handed over to the 82^nd Naval Construction Battalion, which was joined by the 88^th Naval Construction Battalion in January. The airstrip was subsequently extended to 7,000 by 300 ft (2135 by 91 m).

The 87^th Naval Construction Battalion next turned to construction of wharf facilities to accommodate large ocean-going vessels. Four 6- by 18-ft (1.8- by 5.5-m) pontoon barges were secured to 16-ft (4.9-m) square timber crib piers, which were connected to the shore by ramps fabricated of girders covered with wooden planks. The first ship docked on 30 January 1944. A naval base was developed with workshops, stage facilities and a 100-bed hospital.

PT-boats based in the Treasury islands group helped to protect Allied forces landing at Torokina, while the radar site at Soanotalu played an important part in the success of that operation. The air base was used by the medium bombers of the USAAF’s 42^nd Bombardment Group and the US Marine Corps' VMB-413 squadron, while the base facilities were utilised by the US Navy’s Acorn 12.

Base development was considered complete by July 1944, and responsibility for its use and maintenance was handed over to Construction Battalion Maintenance Units 569 and 587. Some of the base facilities were shipped to Leyte island in the Philippine islands group during December 1944 and January 1945, and the base closed when CBMU 569 departed in June 1945.