Long-term monitoring is a prerequisite to understanding and protecting long-lived species such as cetaceans. In New Caledonia, South Pacific, an endangered sub-population of humpback whales (Megaptera novaeangliae) seasonally congregates for mating and nursing during the austral winter. For more than two decades, dedicated surveys have been conducted at sea and from land to monitor humpback whale presence in a coastal breeding site, the South Lagoon. Methods were developed to investigate space use patterns and their temporal variations over the long term using a joint dataset of boat-based and land-based observations (1995-2017). A total of 2651 humpback whale groups were observed, including 1167 from land and 1484 at sea (of which 30% were initially detected by the land-based observers). Humpback whales displayed a persistent space use pattern over this 23 year period, consistent social composition over the years, and an increase in the group encounter rates from land and at sea. The core area of use by humpback whales was characterized in the austral winter by stable and relatively low sea surface temperature (22°C). Whales consistently occupied nearshore waters from 10 to 200 m deep and open to the ocean. Waters surrounded by dense coral reefs were avoided. Although humpback whale distribution patterns were persistent and occurrence was found to increase over two decades, a mismatch between humpback whale critical habitat and marine protected areas was revealed. In the context of growing anthropogenic pressure from tourism and industrial development, these findings should be incorporated into local management efforts to protect the endangered Oceania humpback whale in one of its main breeding sites.