A water hammer arrester for installation into a water pipeline, the water hammer arrester including an inlet for receiving water from the water pipeline, an outlet for returning the water to the water pipeline, a body connecting the inlet and the outlet, and a piston assembly positioned in the body to allow water to flow from the inlet to the outlet between the piston assembly and the body, wherein the piston assembly is adapted to absorb pressure variations in the water pipeline to thereby at least partially reduce water hammer in the water pipeline in use.