Hemp fibre reinforced unsaturated polyester composites (HFRUPE) were subjected to water immersion tests in order to study the effects of water absorption on the mechanical properties. HFRUPE composites specimens containing 0, 0.10, 0.15, 0.21 and 0.26 fibre volume fraction were prepared. Water absorption tests were conducted by immersing specimens in a de-ionised water bath at 25 °C and 100 °C for different time durations. The tensile and flexural properties of water immersed specimens subjected to both aging conditions were evaluated and compared alongside dry composite specimens. The percentage of moisture uptake increased as the fibre volume fraction increased due to the high cellulose content. The tensile and flexural properties of HFRUPE specimens were found to decrease with increase in percentage moisture uptake. Moisture induced degradation of composite samples was significant at elevated temperature. The water absorption pattern of these composites at room temperature was found to follow Fickian behaviour, whereas at elevated temperatures it exhibited non-Fickian.