'Semut' was a series of Australian reconnaissance operations carried out by the 'Z' Special Unit during the final stages of World War II (March/October 1945).

This undertaking was the part of the Borneo campaign, and was undertaken in Sarawak, in north-western Borneo, in support of Allied operations to secure North Borneo. 'Agas', another closely related operation, was carried out concurrently in North Borneo. Both operations combined and relayed the intelligence they gathered through the Stallion Project to Australian forces and carried out guerrilla warfare against the Japanese in the region with the full support of the local population. A total of four 'Semut' operations was undertaken, concluding in September and October 1945.

Early in the Pacific War, the Japanese had landed in north-west Borneo and quickly captured the area’s vital oilfields, whose restored production to contribute to the Japanese war effort by 1943. Allied efforts to interdict the flow of oil had been limited largely to aerial bombing in the intervening period as Allied ground efforts had focused upon the drive on the Philippine islands group. Planning for covert operations in Borneo by Allied forces had begun shortly after the Japanese had captured north-west Borneo in December 1941. A British intelligence officer, 2^nd Lieutenant P. M. Synge suggested that 'a force of 500 men or more if necessary, skilled in forest-craft, could be raised…from among the local population…and organised into an effective guerilla force.' Synge submitted a proposal for such operations, but by February 1942 the operation had been rejected because it was not feasible at that time.

In July 1942, Tom Harrisson, who had been the leader of an Oxford University Sarawak Expedition in 1932, created a similar proposal that put an emphasis on operations against the Seria oilfields in Brunei rather than Miri oilfields in Sarawak. Meanwhile, Captain D. L. Leach proposed to use former Borneo civil servants to identify and contact locals and Chinese who were still loyal to the Allies, to organise them and to establish three main bases along the Baram river, in the Rejang basin, and upriver on the Rejang river in preparation to support major Allied operations in the area. While these plans were not implemented at the time, throughout 1942 and 1943, British and Australian army planning staffs worked to exchange information and sought out personnel who might be suitable for operations in Borneo. Meanwhile, the Allies established several organisations, such as Special Operations Australia and its military arm, the 'Z' Special Unit, to carry out covert operations in the Pacific.

The 'Oboe VI' operation to capture Labuan and Brunei Bay were scheduled for the middle of June 1945, and to support this, 'Semut' (Malay work for ant) was launched in the middle of 1945 by the Australian Services Reconnaissance Department with two main objectives: to gather intelligence and to train the indigenous people in launching guerrilla warfare against the Japanese. The operation was commanded by Major G. S. Carter, a New Zealander serving in the Australian army, and was divided into three main parties: 'Semut I' under the command of Major Tom Harrisson, 'Semut II' under the command of Carter himself, and 'Semut III' under Captain W. L. P. Sochon, who had previously served as a police officer in Sarawak before the war.All three men therefore possessed experience of the conditions in Borneo, and understood the local culture and possessed some language skills. Japanese troops occupying the area were drawn from Lieutenant General Masao Baba’s 37^th Army.

'Semut I' was to operate in the Trusan river valley and the surrounding areas, 'Semut II' in the Baram river valley and the surrounding areas, and 'Semut III' in the Rajang river Valley. Harrisson and his 'Semut I' party were inserted by parachute into the Kelabit highlands, to operate around Bario, in March 1945; however, upon the completion of small airstrip in Bario by local labour, Harrisson shifted his base to Belawit, which was located in the Bawang river valley in Dutch Borneo. The 'Semut II' party was also dropped by parachute around Bario in the middle of April. After receiving support from the Kelabit people, the team was transferred to the Baram river valley where it established a base at Long Akah. Sochon then moved out from the location of 'Semut II' and led the 'Semut III' party to Belaga at Upper Rajang, with full support from the Kayan and Iban peoples there. All the intelligence from these operations was relayed to General Sir Thomas Blamey’s Advanced Land Headquarters, locate at Morotai island in the Halmahera islands group. The 'Semut II' party captured a Japanese communications station at Long Lama several days before the 'Oboe VI' landings around Labuan and Brunei Bay. On 9 June 1945, on the eve of the Australian landings at Labuan island, the 'Semut I' party attacked a small Japanese garrison on Brunei Bay.

On 26 April, a plan named 'Stallion' was implemented to collect intelligence from 'Semut' and 'Agas' regarding the Japanese positions in and round Brunei Bay. This information was passed by radio to the headquarters of Major General G. F. Wootten’s Australian 9^th Division on Morotai island to support future operations in the area by elements of Brigadier W. J. V. Windeyer’s Australian 20^th Brigade and Brigadier H. S. W. C. Porter’s Australian 24^th Brigade, which landed in North Borneo on 10 June 1945. Information gathered included troop dispositions, identification of transportation routes and staging points, data about Allied prisoners of war in the area, and the locations of Japanese airfields, food supplies and ammunition dumps.

'Semut IV' was split into two serials (IVA and IVB) which operated around Bintulu on the coast after being delivered by sea, with the task of protecting the flank of the other three 'Semut' parties. At Sarawak, between 13 and 23 August, 'Semut IVB' sailed out of Labuan on the commissioned junk Tiger Snake and moored at Mukah. Party leader, Lieutenant Rowan Waddy, and Lieutenant Ron Hoey, paddling folboat collapsible canoes paddled along the Mukah river to engage, with the help of members of the local population, any remaining hostile Japanese groups. After the end of active hostilities in the middle of August, 'Semut' operatives continued to work around Sapong under Harrisson until a time late in October, during which time they secured the surrender of remaining Japanese troops who were engaged in fighting with local Bawang guerrillas. Throughout the operation, the inserted 'Semut' personnel were resupplied from the air by the Royal Australian Air Force’s No. 200 Flight.

Assisted by the 'Agas' and 'Semut' operations, the Australian 9^th Division was able to secure North Borneo, with major combat ending largely by July 1945. As the Australian conventional forces remained confined to the coastal areas, the Japanese moved inland and the irregular forces, particularly those assigned to 'Semut', continued to play a role, calling in air attacks on the withdrawing Japanese and working to restore civil administration. Following the cessation of hostilities, the Australian conventional troops remained in North Borneo to restore law and order, and to facilitate the surrender of Japanese troops. Extensive civic actions began even before the end of the war, with efforts concentrated on the rebuilding of the oil facilities and other damaged infrastructure, establishing schools, providing medical care to local civilians, and restoring the water supply. Martial law was initially imposed, but eventually a civil administration was established under the British Borneo Civil Affairs Unit.

'Semut I' and 'Semut II' area are considered to have achieved remarkable success. In June 1945, the elements of 'Semut I' were spread thinly, covering the entire northern Sarawak region and having an outpost at Pensiangan and as far as Tenom in North Borneo. This party had also established several bases in Dutch Borneo. Sourced solely from the local population, the Australians were able to obtain information regarding Japanese positions and movements in Brunei and northern Sarawak, and information regarding prisoners of war and civilian internees in the region. About 600 members of the local population were trained and supplied with weapons and ammunition. 'Semut I' also carried out operations to focused on disrupting Japanese subsistence operations and prevent the local population from providing labour to them. It has been asserted, probably correctly, that 'Semut I' was responsible for 1,000 of the Japanese forces 1,700 casualties at the coast of 110 white lives. Another assessment places Japanese casualties as a result of 'Semut' at around 1,500 men, of whom 240 were taken prisoner. These were inflicted by a force of around 82 Allied soldiers and 200 local guerrillas. There were no casualties amongst the 'Z' Special Unit members assigned to 'Semut', but about 30 local guerrillas were killed during the fighting.

'Semut II' also expanded its operational area to Bintulu and Upper Rajang in central Sarawak and trained a 350-strong local guerrilla force. 'Semut III' expanded its operational area to Kapit in central Sarawak.

However, it cannot be denied that some of the information relayed by the local population was faulty, and the progress of the Australian forces in the region was hindered by incorrect information about Japanese strength and dispositions. In this regard, it has been concluded that Allied interrogators failed to assess the reliability of the information gathered before passing it to their headquarters. The local population was unable to differentiate facts and opinions from rumours circulating the region and subsequently passed incorrect information to the Australian forces. As a result of Japanese counter-intelligence efforts, European officials could not be inserted into the 'Semut' and 'Agas' intelligence network to supervise the intelligence gathering.