Ten years of photo and associated data records from an extensive fire and vegetation effects monitoring programme established in two large north Australian National Parks were used to (1) develop a simple-to-use semiquantitative fire severity index based on observed fire impact on vegetation, particularly leaf-scorch height; and (2) explore relationships between seasonality and fire severity in different landform and vegetation types. Using a three-tiered fire severity scale, data for 719 fires recorded from 178 plots over the period 1995–2004 indicate that the great majority of early dry season (pre-August) fires were of very low severity (fire-line intensities <<1000 kW m–1), whereas fires later in the dry season were typically of substantially greater severity. Similar trends were evident for vegetation occupying all landform types. The utility and limitations of the fire severity index, and implications for ecologic, greenhouse inventory, and remote sensing applications are discussed.