Extreme environmental conditions can significantly affect the structural integrity of geological materials, with high temperature exposure playing a key role in altering rock mechanical properties. Understanding these effects is crucial for applications in geotechnical engineering and geothermal energy extraction. This study explores the effect of thermal damage on the roughness of extensional cracks in Gondwana Supergroup sandstone subjected to temperatures ranging from 100 to 700 °C. After thermal treatment, the mechanical strength of the sandstone was assessed using Brazilian tensile tests. The roughness of extensional cracks was analysed using Fast Fourier Transform (FFT). Fracture traces were digitized and examined with FFT to extract frequency components of fracture surfaces. From the Fourier Power Spectrum, fractal dimension and Hurst coefficient were calculated, revealing a correlation between increasing temperature and fracture complexity. Results demonstrate that thermal stress increases fracture roughness, with higher fractal dimension values observed at elevated temperatures. Thermal damage promotes the development of microstructural flaws, which in turn influence crack propagation by increasing surface roughness. These findings provide valuable insights for assessing the integrity of fire-damaged rock and improving geothermal reservoir design. The application of FFT-based fractal analysis presents a novel and quantitative method for characterizing thermally induced rock damage.