CROPWAT-CLIMWAT recommended Labuan weather station as a climatic proxy for estimating reference evapotranspiration (𝐸𝑇𝑜) and crop water requirements in Brunei Darussalam. Labuan is an island hence this data might not provide better estimations of evapotranspiration to the Bruneian conditions. This study evaluates the suitability of Labuan weather data for estimating reference evapotranspiration (𝐸𝑇𝑜) in Brunei. Long-term meteorological data from Labuan and 11 other stations across Borneo were compared with data from two Bruneian stations, Pekan Tutong and Sinaut. Statistical analyses, including t-tests of monthly temperature, humidity, wind speed, sunshine hours, and derived 𝐸𝑇𝑜, revealed that Labuan exhibits a distinctly moderated maritime climate, with significant differences (p 0.05) in key drivers of evapotranspiration compared to the Bruneian sites. Labuan’s monthly 𝐸𝑇𝑜 systematically underestimates Bruneian 𝐸𝑇𝑜, with deviations exceeding 14% during critical irrigation periods. No single external Borneo station in the CLIMWAT database provided a statistically suitable substitute for the sites in Brunei. It is found that geographic proximity does not guarantee climatic homogeneity for energy-driven variables like 𝐸𝑇𝑜 , as local topography and continentality create distinct microclimates. It is concluded that the use of Labuan data to estimate 𝐸𝑇𝑜and then irrigation planning for Brunei either over or underestimate those hence it is recommended to use local meteorological data to the Bruneian conditions.