Climate change is intensifying the frequency, duration, and severity of heatwaves globally, posing a growing threat to human health. However, few fine-scale projections of heatwave-related excess mortality account for spatial disparities and adaptive capacity. We aimed to estimate future heatwave-related excess mortality across statistical area level 2 (SA2) communities in Australia under multiple climate scenarios. In this modelling study, we projected excess mortality rates across 2288 SA2 communities in Australia for the period 2020-2100 under four shared socioeconomic pathways (SSPs) representing alternative trajectories of adaptation (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). Daily all-cause mortality data from Jan 1, 2009, to Dec 31, 2019, were obtained from the Australian Coordinating Registry and the Australian Bureau of Statistics. We estimated historical exposure-response relationships using a two-stage distributed lag non-linear model with multivariate meta-regression. Future daily temperatures were obtained from bias-corrected, downscaled projections based on Coupled Model Intercomparison Project Phase 6 (a global climate model intercomparison project) and combined with mortality data and SSP-specific population forecasts to estimate annual excess deaths and excess mortality rates. We assessed median percentage changes in annual excess mortality rates for 2050-59 and 2090-99 using the 2020-29 period as a reference. Two adaptation scenarios were considered: no adaptation and full adaptation. Uncertainty was quantified through Monte Carlo simulations. Heatwave-related excess mortality was projected to increase substantially across Australia under all SSP scenarios. We estimated that, in 2100, without adaptation, annual excess deaths would reach approximately 5820 under SSP5-8.5 (a scenario of a fossil fuel-intensive future with little mitigation) and the cumulative total of heatwave days across all communities would be 174 079. Heatwave-related excess mortality rates were projected to be highest in Northern Territory during 2090-99, at 33·9 deaths per 100 000 population (95% empirical CI 13·9-55·0), followed by Queensland, at 18·4 deaths per 100 000 population (7·6-29·8), and New South Wales, at 12·8 deaths per 100 000 population (5·3-20·7); projected percentage changes in excess mortality rate relative to 2020-29 ranged from 356% (in West Coast, South Australia) to 4412% (in Thamarrurr, Northern Territory). Although full adaptation substantially reduced the projected mortality burdens, considerable residual risks remained. Spatial disparities in excess mortality rates persisted across states, socioeconomic strata, and urban-rural classifications, although absolute differences were modest. This study provides a comprehensive assessment of future heatwave-related excess mortality across Australia under multiple climate change and adaptation scenarios. These high-resolution projections underscore the urgent need for integrated mitigation and locally tailored adaptation strategies to address climate-related health inequities. Australian Research Council and Australian National Health and Medical Research Council.