In this work, valorisation of high acid value waste cooking oil (WCO) into biodiesel has been assessed using supercritical methanolysis. The effect of the water content in the feedstock has been critically investigated. Using supercritical methanolysis, the higher water content in the feedstock enhanced the hydrolysis of triglycerides to free fatty acids (FFAs) and the esterification of FFAs into fatty acid methyl esters (FAMEs) has been reported. The effect of water content has been investigated by adding different volumes of water to the feedstock prior to the reaction. Response Surface Methodology (RSM) using Central Composite Design (CCD) has been used to design the experiments and to optimise the experimental variables. Five controllable reaction parameters have been studied including methanol to oil (M:O) molar ratio, reaction temperature, reaction pressure, reaction time and water content. Biodiesel yield has been chosen as reaction response for the experimental runs. The linear effect of reaction parameters and their interactions on biodiesel yield has been analysed. It has been observed that increasing the water content of the feedstock decreases the yield of biodiesel at specific conditions. However, due to the high interactive effect between water content and reaction time, it has been observed increasing effect at longer reaction time. A quadratic model has been developed using the reported experimental results representing biodiesel yield function in all of the experimental parameters. The adequacy of the predicted model has been checked statistically using analysis of variance (ANOVA). Numerical optimisation has been applied to identify the optimal reaction conditions for maximum production of biodiesel. The developed optimal condition has reported 99.8% biodiesel yield at 10:1 M:O molar ratio, 245 oC, 125 bar, 6 vol% of water content within 19 min.