Productive water use can be an effective adaptation strategy for improving crop performance. A 2-year field study was undertaken in 2018 and 2019 to investigate the effect of sowing date and genotype on water-use efficiency of lentils grown in diverse locations in Australia. Above-ground dry matter accumulation, grain yield, soil evaporation, water use, and water-use efficiency (WUE) were measured and/or calculated at crop maturity. Early sowing (SD1/mid-April), late maturity and supplementary irrigation increased water use. The long growth cycle resulting from early sowing influenced WUE for dry matter production and grain yield. WUE ranged from 10.5 to 18.8 kg dry matter ha−1 mm−1 (WUEET (evapotranspiration)) and 17.1 to 28.3 kg dry matter ha−1 mm−1 (WUET (transpiration)) for dry matter production. For grain yield, WUE ranged from 2.11 to 5.65 kg grain ha−1 mm−1 (WUEET) and 4.71 to 9.19 kg grain ha−1 mm−1 (WUET). There was more water loss through soil evaporation in SD1 compared to the other sowing dates. Excessive or limited availability of water did not translate to more dry matter accumulation and grain yield. The study concluded that SD1 gives the maximum water productivity for biomass, and SD2 (end of April) and SD3 (mid-May) for grain yield.