A mudline riser annulus flow meter includes a liner configured to be attached to a riser to cover a hole; a cover configured to be attached to the riser to cover the liner such that a cavity is formed by the liner and the cover; a sensor rod configured to be attached to the liner and to extend inside the cavity, the sensor rod having a bore; a magnet assembly configured to be fixedly attached to the sensor rod; and a waveguide tube attached to the cover. The bore of the sensor rod is configured to receive the waveguide tube.