BackgroundFirst responders (police, fire and ambulance officers) are at a significantly increased risk of experiencing poor mental health, including depression and post-traumatic stress disorder. These conditions are associated with high rates of cardiovascular disease, in part due to low levels of physical activity (PA) and high levels of sedentary behaviour. Using a person’s social support system may be an effective solution to help increase PA levels to improve mental and physical health outcomes. We will examine the efficacy of a group-based online intervention in increasing PA in first responders and their support partners, iteratively codesigned with advisors with lived experience of mental illness among first responders.MethodsThis study will recruit a convenience sample of self-identified sedentary first responders and their self-selected support partners to a 10-week PA programme delivered through a private Facebook group. We will deliver education on predetermined topics related to PA and diet and provide participants with an activity tracker (Fitbit). A stepped-wedged design will be applied to compare multiple baselines to intervention and follow-up phases within subjects. Five cohorts of n=20 will be recruited, with each cohort randomised to a different baseline length. Our primary outcome will be psychological distress (Kessler-6). Secondary outcomes include feasibility, self-report and objective PA data (Simple Physical Activity Questionnaire and Fitbit accelerometry), depression and anxiety (Depression Anxiety and Stress Scale-21 items), post-traumatic stress disorder symptoms (PTSD Checklist for DSM-5), quality of life Assessment of Quality of Life-6 dimensions, sleep quality (The Pittsburgh Sleep Quality Index), suicidal ideation (Suicidal Ideation Attributes Scale) and social support for exercise. The mobile data collection platform MetricWire will be used.Ethics and disseminationEthical approval was obtained from the University of New South Wales, Deupty Vice-Chancellor Research, Human Research Ethics Committee on 3 June 2019, HC180561. Findings will be published in peer-reviewed journals and disseminated at national conferences.Trial registration numberACTRN12619000877189.