Abstract Extreme heat in New Zealand is projected to intensify faster than mean warming, but the mechanisms remain unclear. Using dynamically downscaled CMIP6 projections, we investigate whether land–surface moisture constraints contributes to future heat‐extreme intensification in a maritime climate. Across six downscaled models, evaporative fraction (EF), a measure of surface energy partitioning, exhibits a robust negative relationship with the hottest day of the year (TXx), with the strongest coupling occurring approximately 2 months prior to TXx days. This highlights the importance of antecedent land‐surface conditions and land‐surface memory in preconditioning extreme heat. Future projections show widespread reductions in EF across New Zealand. These EF reductions are associated with additional TXx amplification of approximately 1°C–2°C across much of the North Island by the late 21st century. In contrast, projected circulation changes show limited spatial consistency across models. Our results demonstrate that land–atmosphere feedbacks can substantially amplify extreme heat even in maritime climates.