Background: Crises and disasters disproportionally impact people with chronic health conditions such as multiple sclerosis (MS). Objective: To assess the impact of the COVID-19 pandemic and the Australian Black Summer Bushfires on health behaviours in people with MS. Methods: People with MS, carers, healthcare and advocacy professionals were recruited online between May-July 2020 for an online survey and telephone interviews. Results: Survey items relating to health behaviours were completed by 113 people with MS, and 18 people with MS, 4 MS advocates, 5 healthcare professionals, and 2 carers were interviewed. The bushfires affected 34.5% and the pandemic affected 74.3% of survey participants with MS. The pandemic and bushfires caused a decrease in physical activity in 53.8% and 55.3% of participants respectively, as well as increases in unhealthy eating (43.6% and 24.3% respectively) and alcohol consumption (35.4% and 10.5% respectively), and a decrease in typical sleeping patterns (40.5% and 39.5% respectively). Conversely, 27.5% of participants reported an increase in physical activity during the pandemic. Interview data detailed the circumstances and motivations for changes in health behaviours, as well as consequences, including reduced mobility, fitness, mood disturbances, and weight gain. Conclusion: There is a need to increase support and health promotion for people with MS to maintain or initiate positive health behaviours, especially in times of adversity.