A broad flight of upper Quaternary marine terraces in South Taranaki is being progressively dissected by fluvial processes. Small terrace-front valleys as long as 1 km are initiated and lengthened by headward sapping. This process is largely controlled by subsurface flow in permeable terrace cover beds overlying impermeable Tertiary mudstones and siltstones. The gentle, seaward dip of wave-cut surfaces results in dominantly shore-normal (consequent) streams in the early stages of drainage-network evolution. Network growth (bifurcation) is also largely controlled by subsurface flow and may explain why many stream junctions in South Taranaki are roughly orthogonal. A point is eventually reached when a terrace surface becomes completely dissected and drainage networks must evolve by different mechanisms.