Compounding environmental stressors exacerbated by global warming are threatening marine ecosystems worldwide. In New Zealand, recent events such as Cyclone Gabrielle and unprecedented marine heatwaves have brought these concerns to light. Sponge communities dominate the mesophotic reefs of the Motiti archipelago in the Bay of Plenty, which are under ‘no-take’ protection and yet were largely unexplored prior to this research. This study utilised remotely operated vehicle footage and taxonomic identification to establish a baseline of the diversity and current state of Motiti’s mesophotic sponge communities, which are sentinels of the health of their environment. Comprehensive analysis revealed a rapid shift in assemblage composition taking place at the time of surveying, potentially towards species more resilient to sedimentation and elevated temperatures. Additionally, this study reports the discovery of new sponge species, extends geographical occurrence records, identifies large-scale necrosis in line with the 2022 Bay of Plenty marine heatwave, and presents the ‘rapid sponge death’ hypothesis. Conservative findings support previous studies that point towards temperate mesophotic sponges being climate change ‘losers’, highlighting the urgent requirement for regular, long-term monitoring of this sponge diversity hotspot to inform management plans. This study also provides a basis for future research endeavours which are now paramount to understand the complex interactions and the extent of change occurring in this functionally important ecosystem.