Introduction: Mental health concerns among young adults are rising globally, with 29% of young Australians experiencing high psychological distress. Nature-based interventions show promise in improving mental health outcomes, however there is limited evidence on their effectiveness and economic value. This study aims to fill this void by examining the wellbeing and mental health effects of the Shepparton NatureScripts initiative, a group-based nature prescribing program based on learning about and practicing Nature Dosing (noticing, connecting with and caring for nature) for young adults with mild to moderate mental illness. A further aim is to determine the program’s economic merit. Method: There were 60 participants overall; 39 who were engaged in a 6-week program of curated nature-based group activities (Nature Dosing) combined with personal NatureWise headspace sessions and 21 comparison group participants. Eudaimonic and evaluative wellbeing, as well as psychological distress, were measured at baseline, immediate post-intervention and 6-month follow-up, for all participants. Cost-benefit analysis was conducted using wellbeing valuation approaches. Results: Significant improvements were observed in eudaimonic and evaluative wellbeing at program exit and also at six-month follow-up for evaluative wellbeing only. Psychological distress decreased significantly. Social connectedness and pro-environmental behaviours also showed positive (non-significant) trends. Benefits were partially sustained at 6-month follow-up, supported in some cases by participants doing the program twice. A conservative benefit-cost ratio of 4.77 was calculated based solely on wellbeing improvements. Discussion: The Shepparton NatureScripts program demonstrates significant wellbeing and mental health benefits as well as strong economic value for young adults with mild to moderate mental illness. This study contributes to the growing evidence base supporting nature prescribing as an effective, accessible and efficient mental health intervention. Future research should explore long-term outcomes and strategies for sustaining benefits through ongoing nature engagement.