Abstract: A barrier module having an elongate hollow base portion adapted to rest on the ground or equivalent surface and to be filled with liquid, preferably water. The module also includes an elongate upper portion disposed above the base portion, and a wall the base portion and the upper portion and forming a barrier to prevent passage of persons therebetween. The upper portion has an outwardly and downwardly curved upper surface extending from one side of the wall means to the other side of the wall means to make gripping of the upper portion difficult, thereby inhibiting scaling of the barrier.