Rising summer heat and more frequent and intense heatwaves impact countless metropolitan regions, including Greater Sydney, Australia. An analysis of historic air temperature measurements (1859–2020) reveals a notable increase in the number of ‘hot’ (≥35 °C) days during austral summers. While in the first 120 years of records 351 hot days were identified, 478 hot days were recorded during 2000–2020 alone. Trajectories of summer heat until 2060 indicate that maximum air temperatures in Western Sydney could be ≥ 35 °C during 160 days.

A second, more granular analysis compared air temperature measurements recorded at 274 urban microsites during the summers of 2019 and 2020 with measurements of official weather stations in Central and Western Sydney. Results revealed that the number of hot (≥35 °C), extreme (≥40 °C), and ‘catastrophic’ (≥45 °C) heat days was markedly greater than those reported by official weather stations. Underreporting of heat was greatest across the Local Government Area (LGA) of Cumberland, where data loggers recorded 32 hot and 15 extreme heat days, compared to 7 hot and 1 extreme heat day recorded by the nearest official station. Based on empirical measurements, a set of novel ‘heat risk’ maps identify suburbs and regions inside LGAs where underreporting of summer heat is high. Findings indicate that communities across Greater Sydney are exposed to more frequent and more intense heat than previously reported. Underreporting of local urban heat results in lower preparedness and thus higher risk of harm to urban populations of Greater Sydney and likely many other metropolitan regions.