Objective This research examines the existing funding model for in-hospital trauma patient episodes in New South Wales (NSW), Australia and identifies factors that cause above-average treatment costs. Accurate information on the treatment costs of injury is needed to guide health-funding strategy and prevent inadvertent underfunding of specialist trauma centres, which treat a high trauma casemix. Methods Admitted trauma patient data provided by 12 trauma centres were linked with financial data for 2008-09. Actual costs incurred by each hospital were compared with state-wide Australian Refined Diagnostic Related Groups (AR-DRG) average costs. Patient episodes where actual cost was higher than AR-DRG cost allocation were examined. Results There were 16693 patients at a total cost of AU$178.7million. The total costs incurred by trauma centres were $14.7million above the NSW peer-group average cost estimates. There were 10 AR-DRG where the total cost variance was greater than $500000. The AR-DRG with the largest proportion of patients were the upper limb injury categories, many of whom had multiple body regions injured and/or a traumatic brain injury (P<0.001). Conclusions AR-DRG classifications do not adequately describe the trauma patient episode and are not commensurate with the expense of trauma treatment. A revision of AR-DRG used for trauma is needed. What is known about this topic? Severely injured trauma patients often have multiple injuries, in more than one body region and the determination of appropriate AR-DRG can be difficult. Pilot research suggests that the AR-DRG do not accurately represent the care that is required for these patients. What does this paper add? This is the first multicentre analysis of treatment costs and coding variance for major trauma in Australia. This research identifies the limitations of the current AR-DRGS and those that are particularly problematic. The value of linking trauma registry and financial data within each trauma centre is demonstrated. What are the implications for practitioners? Further work should be conducted between trauma services, clinical coding and finance departments to improve the accuracy of clinical coding, review funding models and ensure that AR-DRG allocation is commensurate with the expense of trauma treatment.