The impact of heatwaves on human health is well understood; as a chronic^[1] impost at low intensities and for critical impacts^[2,3] at more intense levels. As a consequence national and international effort has been focussed on the development of decision support systems that enable authorities to activate mitigation and response strategies previously developed within communities, supporting infrastructure and health systems.

The Australian Bureau of Meteorology has adopted the heatwave intensity and severity methodology^[4] used in this study which has been validated in Australian and international studies. The excess heat factor (EHF) combines local long and short term temperature anomalies, creating a metric that has a demonstrated sensitivity to adaptive capacity as a function of heatwave intensity. In a second step, the history of heatwave intensity is treated statistically to create severity thresholds that are relevant for each location. Australian and international studies^[5,6] have identified public health impacts where heatwave severity thresholds are linked to adverse health outcomes and degraded supporting infrastructure. In this step the heatwave severity thresholds for ‘low-intensity’, ‘severe’ and ‘extreme’ heatwaves can act as a traffic light system to support activation of heatwave response plans.

We describe the variability of heatwave intensity across Australia whilst taking into consideration the strength and speed of synoptic scale weather systems and soil moisture spatial and temporal variability. Capturing the stability of weather and climate determinants of heatwave severity will provide assurance to users of the Bureau of Meteorology’s heatwave services.