Australia has experienced the longest bushfire season in 2019–2020 summertime. This study investigates the indoor air quality, represented by indoor air temperature, relative humidity, PM2.5 concentration and CO2 concentration, in 16 Brisbane and 5 Gold Coast residential buildings from Dec 2019 to Feb 2020. Results demonstrated that in general, IAQ conditions were not seriously affected by the bushfire, except a few days in December 2019. The indoor and outdoor PM2.5 concentrations were very close during normal building ventilation conditions; however, the indoor to outdoor ratio can range widely between 0.21 and 7.50 during bushfire days under minimal ventilation conditions. Staying indoors with windows and doors closed might not always bring benefits during bushfire events, as internally generated fine particulates might exacerbate the level of pollution indoors.