There is an increasing public and policy awareness that tracking cookies are being used to support behavioral advertising, but the extent to which tracking is occurring is not clear. The extent of tracking could have implications for the enforceability of legislative responses to the sharing of personal data, including the Privacy Act 1988 (Cth). In this paper, we develop a methodology for determining the prevalence of tracking cookies, and report the results for a sample of the 50 most visited sites by Australians. We find that the use of tracking cookies is endemic, but that distinct clusters of tracking can be identified across categories including search, pornography and social networking. The implications of the work in relation to privacy are discussed.