This was an Australian part of a programme of special forces operations by the 'Z' Special Unit in the Brunei Bay area on the coast of Japanese-occupied Borneo (29 May/7 June 1945).
Schemed from 29 April, the undertaking was designed to provide specific information on the terrain and the Japanese dispositions in the immediate hinterland areas of Brunei Bay in preparation for the planned 'Oboe VI' landing. Otherwise known as 'Agas III', 'Stallion Phase IV' was launched on 29 May, when the five-man party was transported in a Consolidated Catalina flying boat and inserted into its operation area late in the afternoon at a position to the south of the Bongawan river. Two days later, the party made contact with a local Chinese man, Ah Lee, who was a friend of a member of the 'Stallion' party, a long-serving worker on the North Borneo Railway and currently the Japanese-appointed station master of Bongawan railway station. This man offered to arrange a meeting between Major F. G. L. Chester, the party’s leader, and Chin Sang, otherwise 'Captain China' and an influential Chinese leader of the area.
After much difficulty, Chester eventually met Chin Sang on 2 June. Chin Sang and Ah Lee refused outright to become involved in operations, but told Chester that they willing to provide information of Japanese troop movements, concentrations and other related matters.
Meanwhile the party noted activities in and around the railway track, such as train schedule, cargo to and from Beaufort (stores and troops respectively), and the collection of timber by railway contractors. As there were many Japanese in the area, an attempt to make contact with Ng Wai Wong, former chief clerk of the Kimanis Estate, proved fruitless, and did the attempt by one man of the party to enter the Papar area. The party’s base had to be moved on four occasions for safety and finally, on 5 June, radio contact was made with base: this had not been possible up to this time as there were Japanese within earshot of the party’s electrical generator. The party returned to base on 7 June.
Though it had been unable to accomplish many of its planned tasks as a result of the strong Japanese concentration in the operational area bounded by Jesselton, Keningau and Beaufort, the intelligence provided by the 'Stallion' party was useful in the planning of 'Oboe VI'. Given the Japanese strength, estimated to be nearly 6,000 troops between Jesselton and Beaufort, post-landing plans for this area were created to meet this Japanese concentration. Apart from the strong Japanese presence, Chester and the men of his party observed that the Chinese in the area refused all physical co-operation for fear of Japanese reprisals, largely because many Chinese guerrillas had been killed during a failed revolt in October 1943. In overall terms, the intelligence gained from this mission prevented a head-on clash with the Japanese.
Later parts of the 'Stallion' undertaking were codenamed 'Gelding', 'Mare', 'Filly', 'Colt' and 'Foal'.