A method, system and apparatus for continuously determining the watercut of production fluid flowing in a pipeline from an oil reservoir. The method includes providing an electrochemical impedance sensor at a fixed location in the pipeline for continuously measuring the impedance value of the flowing production fluid at such location. A sample of the production fluid is obtained from the pipeline to determine the watercut value of the sample. From such watercut values from the sample, a correlation formula is determined of the watercut value of the sample with the measured impedance value at the fixed location. Subsequently the continuously measured impedance values are inputted into the correlation formula to continuously calculate the watercut value of the flowing production fluid to thereby continuously determine the watercut of the flowing production fluid with the impedance sensor.