Educating for sustainability is a critical step in moving towards a more sustainable future for humanity. As higher education moves into the online space, education for sustainability can reach larger and more diverse audiences; the University of Tasmania’s Diploma of Sustainable Living is one such example. However, while sustainability education has the potential to impact student attitudes and behaviours about sustainability, these impacts are rarely evaluated. Here, we present the outcome of a study evaluating the impacts of taking a fully online unit, Backyard Biodiversity (KPZ006), part of the Diploma of Sustainable Living. In this study, we analyse 265 paired, before-after surveys to examine changes in student biodiversity knowledge, attitudes (including connectedness to nature), pro-environmental behaviours, and wellbeing. We found statistically significant increases in students’ subjective knowledge and agency around biodiversity management and sustainability attitudes, including the perceived importance of biodiversity and nature connectedness. Students also reported more pro-environmental behaviours after taking the unit and many believed that taking the unit improved their wellbeing. We attribute these impacts to the unit design, which emphasised nature-based experiential learning designed to facilitate wellbeing benefits. While this study is specific to our online unit and the students who completed the survey, the results suggest that sustainability education—even fully online units—if carefully designed can create real-world impacts for sustainability and student wellbeing.