As life expectancy in industrialised societies is increasing, so too are lifestyle-related health conditions such as obesity and diabetes. These conditions have been associated with health behaviours, for example physical inactivity and sedentary lifestyles, which can have adverse effects on physical and psychological wellbeing. It is therefore important for researchers to continue to explore the mechanisms and processes that may drive these key health behaviours. Guidelines for physical activity suggest adults should engage in a minimum of 150 minutes of moderate-to-vigorous physical activity each week and limit sedentary time to eight hours or less while also breaking up long periods of sitting. Despite this, 49.4% of the Australian population report living a sedentary life, with 44% of people describing their workday as predominantly sedentary. For most adults, much of the day is spent at work, thus making the workplace an opportunistic environment for targeting health behaviour change. Furthermore, given recent societal changes in workplace environments as a result of the COVID-19 global pandemic, it is important to gain understanding of the similarities and differences in the experiences of people working from a range of environments with respect to the target behaviours. Therefore, the overarching aim of this PhD program of research was to explore and test an extended model of the theory of planned behaviour with the inclusion of automatic constructs upon physical activity and sedentary behaviour within a sample of sedentary office workers who were working from a range of environments including commercial office, home office, and a combination of home and commercial office. [...]