Little research has assessed the fidelity, adaptation or integrity of activities implemented within community-based obesity prevention initiatives. To address this gap, a mixed-method process evaluation was undertaken in the context of the South Australian Obesity Prevention and Lifestyle (OPAL) initiative. An ecological coding procedure assessed fidelity and adaptation of activity settings, targets and strategies implemented in the second year of four communities. Implementation integrity reflected fidelity and adaptation to local context, whereas efforts resulting in significant deviations from the original plan were deemed to lack fidelity and integrity. Staff implemented 284 strategies in 205 projects. Results show that 68.3 and 2.1% of strategies were implemented with fidelity or adapted, respectively. Overall, 70.4% of all strategies were implemented with integrity. Staff experienced barriers with 29.6% of strategies. Chi-square analyses show statistically significant associations between implementation integrity and strategy type, intervention and behavioural targets. These relationships are weak to modest. The strongest relationship was found between implementation integrity and proximal target. Staff experienced implementation barriers at the coalition, policy, organization, interpersonal and community levels. The greatest range of barriers was encountered working with organizations. To overcome these barriers, staff took greater ownership, invested more time, persisted and allocated more financial resources.