Tidal velocity asymmetry at inlets influences sediment transport pathways and the morphological evolution of estuaries/lagoons connected to these inlets. Generation of overtides is generally seen as the main cause of tidal velocity asymmetry. Whilst majority of studies examining tidal velocity asymmetry have concentrated on inlets located in semi-diurnal tidal regimes, here, attention is focused on the processes responsible for causing tidal velocity asymmetry at inlets located in diurnal tidal regimes. Using field data collected from three West Australian inlets, it is shown that tidal velocity asymmetry in this type of system is caused by the oceanic tidal conditions. It is also shown that in these systems, the occurrence of flood/ebb dominance can be determined using oceanic tidal elevations, which are more readily available than inlet current data. In contrast to semi-diurnal systems the flood/ebb dominance in diurnal systems varies throughout the year depending on the phase angle relationship between the significant oceanic tidal constituents. The net sediment transport in to/out of these systems, which determines the morphological evolution of the systems, is shown to be governed more by the degree of tidal velocity asymmetry rather than the number of occurrences or duration of flood/ebb-dominant periods. (C) 2000 Published by Elsevier Science Ltd.