A drainage system comprises an elongate flashing member, to be received beneath a course of CMUs, in use. A layer of water permeable material is attached to an upper surface of the flashing member. The water permeable material includes a longitudinal portion extending longitudinally adjacent a rear edge of the flashing member and a plurality of transverse portions extending transversely from the longitudinal portion to a front edge of the flashing member. A plurality of blocks of water permeable material are provided, each being positioned above the layer of water permeable material and extending upwardly into cavities of CMUs, in use. The water permeable material of the layer and the blocks has a porosity sufficient to permit water to pass therethrough but substantially insufficient to permit mortar and debris to pass therethrough so that water in cavities of the CMUs drains through the transverse portions.