An extension plate assembly for dumping debris into a dump box from a storage tank mounted on a trunk. The storage tank is fitted with a pusher plate that both compacts and pushes debris out of the back of the storage tank. The extension plate apparatus includes extension arms that are rotatably attached on one end to the pusher plate and rotatably attached on the other end to an extension plate. When the pusher plate is in its rearward most position, the extension arms extend beyond the edge of the storage tank. The extension plate assembly allows debris to be pushed out of the storage tank and distributed more evenly into a dump box.