Background: It is important to quantify the health impact of heatwaves. However, little is known about the geographic difference of the heatwave - mortality relationship in the southern hemisphere. This study assessed the impact of heatwave on mortality in three major Australian cities: Brisbane, Melbourne and Sydney. Methods: Daily data on climatic variables and mortality for these three cities during the period 1988 to 2009 were obtained from relevant government agencies. A consistent definition of a heatwave was used for these cities. Generalized additive Poisson regression models were fitted to assess the impact of heatwaves on mortality after adjustment for confounders. Results: Total mortality increased mostly within the same day (lag 0) or a lag of one day (lag 1) during almost all heatwaves in three cities. Using the 95 percentile of mean temperature for two or more consecutive days in the summer season as a consistent heatwave definition (HWD) across different cities, the relative risk for total mortality at lag 1 in Brisbane, Melbourne and Sydney was 1.13 (95% confidence interval (CI: 1.08 -1.19), 1.10 (95% CI: 1.06 -1.14) and 1.06 (95% CI: 1.01 -1.10), respectively. Using the more stringent HWD, i.e. the 99 percentile of mean temperature for two or more consecutive days in the summer season, the relative risk of total mortality at the lags of 0-2 days in Brisbane and Melbourne was 1.40 (95% CI: 1.29 -1.51) and 1.47 (95% CI: 1.36 - 1.59), respectively, indicating significant increases in mortality risk. Elderly, particularly females, were more vulnerable to the impact of heatwaves. Conclusions: A consistent and significant increase in mortality was observed during heatwaves in three major Australian cities, but the impacts of heatwave varied with age, gender, the HWD and geographic area.
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