Greenwashing is a threat not only to consumer trust but also to the integrity of planetary health efforts, where transparency in sustainability claims is critical for promoting ecological wellbeing, food security, and equitable development in the Anthropocene. This paper investigates greenwashing by adapting Gomper’s Governance Index methodology to the context of sustainability claims. The focus of our greenwashing index in this case is the sustainability claims made by canned tuna brands in Australia. The index is created from a comprehensive set of criteria on environmental claims, based on the Australian Competition and Consumer Commission (ACCC)’s principles for trustworthy claims. We show that canned tuna brands formed two clusters: one at a very high level of achievement, and a second group with notable opportunities to improve on their sustainability communication and transparency. The results also highlight key issues, including an absence of information on future sustainability transition plans across most brands. A deeper analysis of the scoring scheme shows that the brands which attained third-party sustainability certification, generally achieved a better alignment with the ACCC principles than other brands. Future iterations of this analysis could incorporate online transparency and third-party verification to provide a more comprehensive assessment. Overall, this study underscores the need for clearer sustainability messaging, potentially greater regulatory enforcement, and improved accountability among brands to ensure consumers can make informed choices.