A wide range of microbial and chemical characteristics in drinking water have the potential to affect human health. However, it is not possible or practical to test drinking water for all potentially harmful characteristics. If drinking water is contaminated, people may already be exposed by the time test results are available. The 'boil water alert' issued in Sydney, Australia in 1998 following the detection of Cryptosporidium and Giardia in the finished water supply, highlighted the uncertainties associated with the public health response to test results. The Sydney experience supports the international consensus that a preventive risk-management approach to the supply of drinking water (manifesting as water safety plans (WSPs)) is the most reliable way to protect public health. A key component of a comprehensive WSP is that water suppliers and health authorities must have plans to respond in the case of water contamination and/or outbreaks. These plans must include clear guidance on when to issue warnings to consumers, and how these warnings are to be communicated. The pressure on health authorities to develop clear and systematic boil-water guidance will increase as utilities all over the world develop their WSPs.