Before 1909, Kuala Belait was a small fishing village with little development. Although oil exploration began in the Belait District as early as 1911, and some
oil was found in
Labi in 1914, it had minimal impact on Kuala Belait's growth at that time. However, the discovery of oil in Labi convinced prospectors that further deposits might be found along the higher banks of the Belait River. That same year, a 60-mile (97 km) road was constructed, linking Kuala Belait to
Brunei Town. Oil companies began concentrating their activities in the area, leading to the emergence of a thriving village in Kuala Belait by 1927. At the time, it served as a port for transporting machinery crucial to oil prospecting, primarily managed by the
British Malayan Petroleum Company (BMPC). Kuala Belait soon became a key entry point to the upper reaches of the Belait River, and the government recognised its growing significance by establishing it as the headquarters of the Belait District that same year. In 1929, BMPC followed suit by relocating its headquarters to Kuala Belait, further boosting the village's development. The discovery of
large oil reserves in Seria, in April 1929, significantly increased Kuala Belait's importance as a strategic hub for the oil industry. During the oil boom, the position of
Assistant British Resident was reinstated and based in the town to oversee British oil interests, marking the town's growing administrative significance in Brunei's petroleum sector. In that same year, Kuala Belait became a town after the establishment of the Kuala Belait Sanitary Board, which is run by the Assistant Resident. The 1930s marked a period of rapid growth for Kuala Belait, largely driven by the BMPC. In 1931, the town became the site of Brunei's first English school, and it also boasted the best hospital in the country at the time. Its population grew significantly, from 1,193 in 1931 to 3,000 by 1935, fueled by the thriving oil industry. Despite this progress, the town was heavily bombed during
World War II, leading to widespread destruction and the need to rebuild. The BMPC also played a crucial role in infrastructure development, constructing a telephone line along the
Belait coastline earlier in 1930, linking Seria and
Rasau to their headquarters in Kuala Belait. However, a telephone line connecting Kuala Belait to
Tutong was dismantled in 1934 after failing to meet expectations. By 1939, pipelines and roads between Kuala Belait and
Miri had been established, further enhancing the town's connectivity and industrial growth. under the supervision of an
Australian soldier in 1945|200x200px On 16 December 1941, Kuala Belait was captured after an amphibious assault on the Belait coast by 10,000 soldiers from the
Japanese Kawaguchi Detachment. The town remained under
Japanese occupation during World War II. During this time, war crimes were committed by the Japanese, including the massacre and execution of Indian
prisoners of wars (POW) from the
2/15th Punjab Regiment. Additionally, 55 Indian prisoners died from starvation in the town's
POW camp. As part of Operation Oboe Six, the Australian 9th Division arrived in Kuala Belait on 24 June 1945, leading to the recapture of the port. In 1949, reconstruction plans for the war-torn town were approved. In 1950, concerned about the potential spread of
Chinese Communist ideology among Kuala Belait's urban working-class population, the Brunei government relocated the police headquarters from Brunei Town to Kuala Belait and established both a Criminal Investigation Department and a
Special Branch to enhance security. The town experienced significant growth during the 1950s and 1960s, spurred by increased revenue from the booming oil industry. Post-war development transformed Kuala Belait, with the construction of reinforced concrete shophouses, government offices,
godowns, and a wharf, alongside improvements to roads and health facilities that modernised the town by the early 1950s. In 1952, plans were initiated to build a
country palace for
Sultan Omar Ali Saifuddien III in Kuala Belait—he had previously stayed at the rest house during visits—highlighting the town's rising status and the Omar Ali's growing prestige. The same period saw the upgrade of the Sanitary Board to a Municipal Board in 1953. However, British experts expressed scepticism about large-scale infrastructure projects, particularly the proposal to build a modern 175-bed hospital, arguing that the town's population would likely decline after the oil boom. They warned that such a facility might become a white elephant and instead recommended a temporary hospital, while questioning the broader value of heavy investment in the area. The discovery of the first offshore
gas field,
South West Ampa, located off Kuala Belait in 1963, further boosted the town's growth. During the 1962
Brunei revolt,
TKNU rebels briefly gained control of the town, but the town was soon liberated by the 1/2nd Battalion
Gurkha Rifles Regiment, with
local police successfully defending the police station. After Brunei's independence in 1984, several new government buildings were constructed to house local services. In the 1990s, the coastal road from
Muara to Kuala Belait was upgraded to a four-lane highway, further enhancing the town's infrastructure. The Municipal Board was overseen by the Belait District Officer from 31 October 1985. From 1 November 1985, the municipality has been administered by a chairman. By 1997, it had become Brunei's second-largest town, following Bandar Seri Begawan. Often referred to as a "Shell town," much of its postwar development was attributed to Shell's efforts. == Governance ==