The diffuse light attenuation coefficient for the downwelling plane irradiance, Kd (m-1), can be used to estimate light intensity at depth and explore the variability of light available for benthic organisms. The variation in Kd is wavelength-dependent and is determined by the optically active constituents in the water column, including phytoplankton (chlorophyll-a concentration, Chl-a), coloured dissolved organic matter (CDOM), and suspended non-algal particulate (NAP) materials. We used random forest models to understand how these constituents and other environmental predictors affect spectral light attenuation and hence light availability within the Great Barrier Reef (GBR) shelf region. We found that Kd variability was consistent with the optical properties that reflect (i) terrestrial and anthropogenic inputs (CDOM and NAP) in the coastal water bodies and (ii) variations in Chl-a consistent with clear oceanic conditions in the offshore waterbody. In the mid-shelf, variations in Chl-a were also the best predictor of Kd variability, which may reflect the combined influence of land-derived nutrient inputs and local oceanography within the shelf. In optically complex coastal waters like the GBR, information on drivers of light variability is crucial to better quantify and manage the pressures affecting their vast and vital benthic marine ecosystems, including corals and seagrasses.