This work presents an experimental investigation into the effect of cornhusk fibre (CHF) content upon the mechanical properties, water absorption behaviour, and swellability of CHF/polyester (PE) composites used in water environments. The CHF/PE was prepared at different volume fractions using hot compression (~175 °C). To investigate the rate of water absorption and swellability behaviours, composites were immersed in water for varying durations. The mechanical properties of composites (i.e. tensile, bending and compression strengths) immersed in water were carefully evaluated. The results indicate that the composites with an increased CHF content and a longer immersion time are prone to lower mechanical properties. The large amount of water absorbed by the composite reduces the bonding interface between CHF and PE, which is responsible for the damage. Moreover, the amount of water absorbed and the swellability increase with a corresponding increase in the CHF content. The lowest water absorption (2.39%) was detected in 20% CHF and 80% PE composite immersed for 6 days. The findings gathered in this research endorse CHF/polyester thermoset composites as a viable alternative for construction applications.