Objective: This study examined the parallel forms reliability of a web application version (app) of the Activity Card Sort Australia for adults aged 18-64 and assessed clinical utility. Methods: Forty eight participants completed the app and card versions of the tool within a 2to 3-week interval and provided feedback via a purpose-designed survey. Intra-class correlation analysis tested parallel forms reliability. Results: The app demonstrated acceptable parallel forms reliability for the overall retained activity level (r = 0.75, p < 0.001), daily life domain (r = 0.77, p < 0.001), and recreation and relaxation domain (r = 0.74, p < 0.001) but not the physical activity domain (r = 0.59, p < 0.001). Clinical utility responses suggested good acceptance of both versions. Conclusion: The results suggest that further studies are required before the app version can be used for research or in clinical settings.