Background: To provide epidemiological data for minor and moderate-to-serious injury claims for gym and fitness related injuries amongst those aged 16–64 in New Zealand, to inform the development of an injury prevention program. Methods: Retrospective analytical review of gym and fitness related injury entitlement minor and moderate- to-serious Accident Compensation Corporation (ACC) claims from 1 July 2011 to 30 June 2020. Data were analysed by cause of injury, geographical region, sex, age, body site and injury type. Qualitative analysis of free text describing the activity causing the injury was conducted. Results: Over the ten-year period, 16–64 year olds made 345,254 injury claims, costing ACC NZ$241,298,275 in treatment charges. Soft tissue injuries were the most prevalent making up 96% (331,343) of all claims and 88% (NZ$213,049,197) of the total charges. Strenuous movement with lifting (n = 154,467, 47%), strenuous movement without lifting (n = 84,469, 25%), impact/contact with object (n = 39,610, 12%) and impact/contact with ground (n = 25,351, 8%) were the top four mechanisms resulting in injury, accounting for 92% of soft tissue injuries. Males and females aged 21 to 30 years old were most frequently injured. The four most injured body sites (lower back/spine, shoulder, knee, neck/back of head) accounted for 63% of injuries in females, and 65% in males. Conclusions: The most common cause of injury from gym and fitness activity claims in 16–64 year olds in New Zealand was lifting/carrying/strain resulting in lower back/spine and shoulder (including clavicle/blade) soft tissue injuries. Soft tissue injuries accounted for 96% of the total claims. Males and females aged 21 to 30 years old were most frequently injured age group.