Objectives We sought to extrapolate the long-Term costs and clinical impacts attributed to the rugby fans in training-New Zealand (RUFIT-NZ) trial in Aotearoa, New Zealand. Design A modelled cost-effectiveness analysis using efficacy data from RUFIT-NZ was conducted from the Aotearoa New Zealand healthcare perspective. Setting A Markov cohort model was constructed with a lifetime time horizon. The model simulated events of myocardial infarction (MI), stroke and type 2 diabetes mellitus (T2DM) occurring among a hypothetical cohort of 10 000 individuals receiving either the RUFIT-NZ intervention or no intervention. Efficacy data were based on the RUFIT-NZ trial, and the latest Global Burden of Disease study was used to extrapolate the impact of body weight reduction on clinical outcomes of T2DM, MI or stroke. Cost and utility data were drawn from the RUFIT-NZ trial and published sources. Primary outcome measures The incremental cost-effectiveness ratio (ICER). Results Over a lifetime time horizon, participants in the RUFIT-NZ intervention gained 0.02 (discounted) quality-Adjusted life years (QALYs) at an additional cost of NZ$863, relative to no intervention. The estimated ICER was NZ$49 515 per QALY gained (discounted), which is above the arbitrary willingness-To-pay threshold of NZ$45 000 per QALY. Sensitivity analyses supported the robustness of these findings. Conclusions RUFIT-NZ was associated with a reduction in cardiovascular and endocrine events for overweight and obese males. However, based on conservative assumptions, RUFIT-NZ was unlikely to be cost-effective from a healthcare system perspective. Trial registration number ACTRN12619000069156.