Extreme heat events can have serious impacts on wildlife, and this is especially concerning as the frequency, intensity, and duration of these events are expected to increase with ongoing climate change. In Australian flying-foxes, Pteropus spp., heat stress-related mass mortality is highly correlated with exposure to air temperatures ≥ 42°C. Young Grey-headed Flying-foxes (Pteropus poliocephalus) are dependent on their mothers for milk during their first 3 months of life, and this coincides with the austral summer. Extreme heat events particularly affect young individuals and lactating females, and so these events can have disproportionate effects on recruitment and the effective breeding population – two key parameters for conservation. In this study we provide a detailed account of an extreme heat event that occurred at a permanently occupied Grey-headed Flying-fox roost in Campbelltown, NSW. This account includes documentation of animal movements during the extreme heat event and its aftermath, reports on the weather conditions throughout the day, and provides two independent methods for estimating the number of individuals that died from the extreme heat event. Additionally, a sprinkler trial was conducted at the focal roost to test the efficacy of sprinklers in heat stress mitigation, and we provide an assessment of how the sprinklers affected the microclimatic conditions of the roost during extreme heat conditions. This extreme heat event resulted in the death of flying-foxes at the focal roost and at other flying-fox roosts in the greater Sydney and Hunter regions. This dataset contains 3 files that pertain to weather data recorded on the 09th December 2023: two .csv files that include the data from a) weather stations, and b) data logger arrays, and one .txt file that describes the variables in each of the csv files. The “WEATHERSTATION_DATA.csv” file represents the data collected using Davis Pro 2 weather stations that were installed at the two sprinkler sites (Site A and Site B) and a control site that was not subject to the sprinklers (Control). The “ARRAY_DATA.csv” file represents the data collected using data loggers deployed at ten locations within the vegetation occupied by flying-foxes. At each of the ten locations, data loggers were installed at four vertical heights (0, 1.5, 3 and 6 m), for a total of n = 40 data loggers. Kestrel Drops (D2) were used (Kestrel, AU), and housed in plastic Stevenson’s screens. Kestrel D2 Drops (Ta resolution: 0.1° C; RH: 0.1%; dewpoint = 0.1° C) were calibrated in an incubator at four temperature points (15, 25, 35 and 45° C) against a standardised high precision digital thermometer (Platinum Ultra-Accurate Digital Thermometer, Model 6413CC, Traceable, Texas, USA). A linear regression was fitted to explain measured temperature as a function of logger temperature, and the intercept and slope of this equation was used to adjust the data for each logger.