This paper reports on a combined experimental and numerical study dedicated to barrier reefs hydrodynamics. A network of pressure sensors and velocity profilers has been deployed for more than two months over the Ouano reef barrier, New Caledonia. The primary aim of the study is to assess the relevance of the classical depth‐averaged momentum balance in such a complex and poorly documented environment. The combined analysis of experimental and numerical measurements reveals a specific hydrodynamic behaviour contrasting with sandy beaches and fringing reefs. The cross‐reef current induced by wave breaking over the barrier reef plays an important role in the momentum budget, in particular through friction processes. The hydrodynamic behaviour over the barrier reef is thus characterized by the progressive transition from a nearly classical beach type behaviour on the forereef, where the gradient of radiation stress is balanced by a barotropic pressure gradient associated to the wave setup, to an open‐channel type regime, dominated by frictional head loss. The reef top wave setup shows a clear depth‐dependency mainly attributed to the forereef curvature. During extreme wave events, the measurements tend to indicate a transition toward a critical hydraulic regime above the reef top. The numerical simulations, involving a non‐hydrostatic wave‐resolving model coupled to a K‐ϵ turbulence model, highlight the vertical structure of the flow. Over the reef flat, a classical log‐layer profile is observed, in agreement with measurements, while above the forereef an anti‐clockwise circulation develops under the breaking zone.