The quantification of heat and mass flow between deep reservoirs and the surface is important for understanding magmatic and hydrothermal systems. Here, we use high-resolution measurement of carbon dioxide flux (φCO₂) and heat flow at the surface to characterize the mass (CO₂ and steam) and heat released to the atmosphere from two magma-hydrothermal systems. Our soil gas and heat flow surveys at Rotokawa and White Island in the Taupō Volcanic Zone, New Zealand, include over 3000 direct measurements of φCO₂ and soil temperature and 60 carbon isotopic values on soil gases. Carbon dioxide flux was separated into background and magmatic/hydrothermal populations based on the measured values and isotopic characterization. Total CO₂ emission rates (ΣCO₂) of 441 ± 84 t d⁻¹ and 124 ± 18 t d⁻¹were calculated for Rotokawa (2.9 km²) and for the crater floor at White Island (0.3 km²), respectively. The total CO₂ emissions differ from previously published values by +386 t d⁻¹ at Rotokawa and +25 t d⁻¹ at White Island, demonstrating that earlier research underestimated emissions by 700% (Rotokawa) and 25% (White Island). These differences suggest that soil CO₂ emissions facilitate more robust estimates of the thermal energy and mass flux in geothermal systems than traditional approaches. Combining the magmatic/hydrothermal-sourced CO₂ emission (constrained using stable isotopes) with reservoir H₂O:CO₂mass ratios and the enthalpy of evaporation, the surface expression of thermal energy release for the Rotokawa hydrothermal system (226 MW_t) is 10 times greater than the White Island crater floor (22.5 MW_t).