To evaluate fugitive emissions from open cut coal mines, emission factor values of 3.2 m3/t and 1.2 m3/t have been used for the two main Australian coal-producing states of New South Wales and Queensland, respectively. CSIRO developed these values in the early 1990s. They were meant for use as average regional values (Tier 2 method), but were subsequently used for all mines, irrespective of the level of ‘gassiness’ of specific coal seams and strata. Over the past decade, A new method has been developed for Australian open cut mining that is specific to each mine site (Tier 3 method). The proposed method has been adopted by National Greenhouse and Energy Reporting and is the basis of Method 2 or 3 for calculation of emissions. The new method is based on an emission model, which considers the coal seams and sedimentary gas-bearing horizons (layers) as individual gas reservoir units. These units release part or all of their gas during mining. The main data required are in situ gas content, gas composition and thickness of the gas-bearing horizons within the column of strata above and below the mine base. In this method, drilling can be reduced by partitioning the mine site into ‘gas zones’ in which similar patterns of gas distribution are expected. Two to three core drillings are required to characterise a gas zone and to provide the main input of the model. Routine geophysical log data can also provide the thickness of gas-bearing layers. Because of the limitations of the standard gas content measuring method, different commercial laboratories claim various limits of detection (i.e. measurability). However, in view of the very different global warming potential values of coal seam gas components, different limits of measurability can lead to significant differences in the estimation of fugitive emissions.