Privacy Assessment of De-identified Opal Data: A report for Transport for NSW

We consider the privacy implications of public release of a de-identified dataset of Opal card transactions. The data was recently published at https://opendata.transport.nsw.gov.au/dataset/opal-tap-on-and-tap-off. It consists of tap-on and tap-off counts for NSW's four modes of public transport, collected over two separate week-long periods. The data has been further treated to improve privacy by removing small counts, aggregating some stops and routes, and perturbing the counts. This is a summary of our findings.