This article examines privacy threats arising from the use of data mining by private Australian health insurance companies. Qualitative interviews were conducted with key experts, and Australian governmental and nongovernmental websites relevant to private health insurance were searched. Using Rationale, a critical thinking tool, the themes and considerations elicited through this empirical approach were developed into an argument about the use of data mining by private health insurance companies. The argument is followed by an ethical analysis guided by classical philosophical theories - utilitarianism, Mill's harm principle, Kant's deontological theory, and Helen Nissenbaum's contextual integrity framework. Both the argument and the ethical analysis find the use of data mining by private health insurance companies in Australia to be unethical. Although private health insurance companies in Australia cannot use data mining for risk rating to cherry-pick customers and cannot use customers' personal information for unintended purposes, this article nonetheless concludes that the secondary use of customers' personal information and the absence of customers' consent still suggest that the use of data mining by private health insurance companies is wrong.