Climate change projections indicate a likely 3.8°C increase in the average temperature in Australia by 2090. During summer, outdoor heat-stress causes significant thermal discomfort, altering outdoor living preferences. This paper aims to explore the neutral and critical thresholds for outdoor thermal adaptation. The paper reports on outdoor activity change during different outdoor microclimates in Darling harbour, Sydney. Results indicate that outdoor participants adjust their insulation and activity rate by an outdoor neutral thermal threshold of 28-30°C. For the thermal environments hotter than this neutral threshold, the pattern of adaptation shifts towards modification and dismissal of optional, social and ultimately necessary activities. Such thermal adaptation continues to occur by an outdoor critical thermal threshold of 28-48°C. After the critical thermal threshold, outdoor activity prevention becomes the major thermal adaptation strategy. Therefore, to support usability of public spaces during summer heat-stress conditions, public spaces are required to provide thermal environments closer to neutral and no hotter than critical thermal thresholds.