A series of laboratory experiments were conducted to study the wave field in the inner lagoon excited by ‘long’ incident waves. Three cases were considered: Cases A, B and C presenting incident waves of wavelength with factors of 1, 2 and 2.5 times the width of the reef respectively. The reef is of a comparable length to the lagoon region and the reef height is approximately half the water depth, held constant everywhere across the cross-shore domain. For Case A, it is observed that the standing wave formation is ‘trapped’ in the lagoon-reef region, with a succession of nodes and anti-nodes spaced out between each other by approximately half the incident wavelength. The standing wave across the reef is very pronounced, due to the resonance from incident waves with wavelengths of the exact length scale as the reef. The offshore region presents a single formation of a node, entailing that the wave energy is trapped in the reef-lagoon area, with insignificant wave energy reflected offshore. This trapped standing wave in the reef-lagoon region leads to a more important influence of higher harmonics within that region. Case B presented a more complex wave formation, with the highest wave amplitude captured at a location half-way into the lagoon region, higher than the end-wall run up. Very strong higher harmonic influence was captured for Case B in the lagoon region, especially with the 2nd and 3rd harmonic components. It is speculated that the energy transferred from the fundamental frequency component to the higher harmonics is due to the inability of the existing lagoon length to support the formation of standing waves for the fundamental frequency component. Slight wave height attenuation is also observed for this wave condition. Case C presented very distinct peaks (anti-nodes), spaced out from each other by approximately half the incident wavelength. The shape of the wave amplitude envelope clearly demonstrates the fact that the wave is long enough so that the standing wave does not significantly feel the influence of the reef. This is supported by the dominance of the fundamental frequency component across a significant majority of the cross-shore domain. Slight wave height amplification is observed for this long wave case. All three cases were validated with COULWAVE and Celeris, which both solve the extended Boussinesq equations. Better agreement was seen in the case of Celeris.