Objectives To assess the heterogeneity of heatwave-related impacts on mortality across different cities.

Design A multicity time series study.

Setting 3 largest Australian cities: Brisbane, Melbourne and Sydney.

Participants All residents living in these cities.

Main outcome measures Non-external causes mortality data by gender and two age groups (ie, 0–75 and 75+) for these cities during the period 1988–2009 were obtained from relevant government agencies.

Results Total mortality increased mostly within the same day (lag 0) or a lag of 1 day (lag 1) during almost all heatwaves in three cities. Using the heatwave definition (HWD) as the 95th centile of mean temperature for two or more consecutive days in the summer season, the relative risk for total mortality at lag 1 in Brisbane, Melbourne and Sydney was 1.13 (95% CI 1.08 to 1.19), 1.10 (95% CI 1.06 to 1.14) and 1.06 (95% CI 1.01 to 1.10), respectively. Using the more stringent HWD—the 99th centile of mean temperature for two or more consecutive days, the relative risk of total mortality at the lags of 0–2 days in Brisbane and Melbourne was 1.40 (95% CI 1.29 to 1.51) and 1.47 (95% CI 1.36 to 1.59), respectively. Elderly, particularly females, were more vulnerable to the impact of heatwaves.

Conclusions A consistent and significant increase in mortality was observed during heatwaves in the three largest Australian cities, but the impacts of heatwave appeared to vary with age, gender, the HWD and geographical area.