We aimed to determine whether adherence to the Australian dietary guidelines and an index of healthy behavior was associated with a lower risk of type 2 diabetes (T2D) and to provide estimates of the proportion of preventable cases. Participants of the AusDiab cohort study were followed for 12 years (n = 6242), starting from May 1999, during which T2D cases were identified. The associations between T2D risk and a score of adherence to the dietary guidelines, its components, and a score of adherence to an index of healthy behaviors, (which included smoking, recreational physical activity, waist circumference and adherence to the dietary guidelines), were estimated using Cox proportional hazards ratios (HR) and 95% confidence intervals. The proportion of preventable cases was estimated using the population attributable fraction (PAF). Strong adherence to the dietary guidelines was not associated with T2D risk (HR = 0.64 [95% CI 0.39-1.06]), unless moderate alcohol consumption was considered as beneficial instead of no alcohol consumption (HR = 0.59 [0.36-0.96]). However, strong adherence to the guidelines regarding fruit and dairy intake were both associated with decreased risk of T2D (HR = 0.68 [0.51-0.91]; 0.56 [0.38-0.84], respectively) and could have prevented 23-37% of cases (PAF = 23.3% [7.3-38.2]; 37.1% [14.6-56.0], respectively). Strong adherence to the index of healthy behaviors was associated with decreased risk of T2D (HR = 0.30 [0.17-0.51]) and estimated to prevent almost 60% of T2D (PAF = 59.4% [34.3-76.6]). More than half of T2D cases could be preventable in Australia through modifying health behavior. These results could serve as a basis for prevention programs based on lifestyle modification.