This paper reports a combined observational and numerical study of wave transformation over barrier reefs. The field instrumentation, which consists in a cross-shore network of pressure sensors and one high resolution velocity profiler, has been deployed for more than two months over the Ouano reef barrier, New Caledonia. The combined analysis of observations and numerical simulations shows that the reef barrier acts as an efficient, but depth-dependent, filter for incoming wave energy. The data analysis reveals the presence of very low frequency standing wave patterns due to reflection either at the reef barrier inner boundary or at the lagoon shore. Another striking feature is the development of undular bore trains over the reef flat.