  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.
