The Coral Sea hosts a large proportion of the global Wedge-tailed Shearwater Ardenna pacifica population and the few dozen coral islets of the southwestern lagoon of New Caledonia harbour an estimated 470 000 breeding pairs, a significant part of it. Thus, collectivities in New Caledonia have a particular responsibility in the conservation of this species. The census of seabird colonies is an important tool in the management of the corresponding habitats – or so should be. The present work aims at reporting a census of the Wedge-tailed Shearwater colony of M’Ba Islet, the largest coral islet of New Caledonia’s southwestern lagoon. This work is part of an ongoing effort to thoroughly census colonies of the Wedge-tailed Shearwater in New Caledonia. Petrel burrows on M’Ba Islet were counted along 14 transects 4-meter wide and crossing the width of the islet. The transects were distant from one to the next by 50-meter intervals. Each transect was subdivided into 20-meter long segments. The surface sampled was 22 960 m², representing 7.3 % of the vegetated area of M’Ba islet. Two strates of habitat were distinguished according to tree density: the dry forest, covering 246 600 m² and a looser-vegetated, more-recently accreted peripheral habitat dominated by Bay Cedar Suriana maritima shrubs (67 600 m²). Average burrow density was 0.28 m-². Higher values were observed in the core area covered with sclerophyll forest (mean: 0.32 ± 0.06 m-²); lower values, in the peripheral area with looser vegetation (mean: 0.13 ± 0.05 m-²). The total estimate of colony size was 87 971 ± 16 373 burrows, making it the largest colony of the southwestern lagoon of New Caledonia. The quasi-totality of the burrows were assumed to belong to Wedge-tailed Shearwaters. However, a small proportion of the burrows may belong to Black-winged Petrels Pterodroma nigripennis or Tahiti Petrels Pseudobulweria rostrata. M’Ba islet harbours over 5% of the global adult population of the Wedge-tailed Shearwater designating it as an important breeding area for the conservation of the species under international criteria. M’Ba Islet is located only a dozen km off New Caledonia’s largest conurbation (186 000 in-habitants) and is threatened by developing leisure activities. The present results emphasize the need to effectively protect M’Ba islet and other so-far unprotected Wedge-tailed Shearwater colony sites in New Caledonia, which are important for the conservation of this species. We also recommend a census of the populations of Black-winged Petrel and near-threatened Tahiti Petrel breeding on M’Ba Islet.