The idea of using law and regulation to prevent obesity in Australia is complicated by federalism. This article analyses in detail the powers of Commonwealth and State governments to determine which level(s) of government would be able to pass laws of the types recommended by the National Preventative Health Taskforce, namely marketing regulation, labelling regulation, content regulation, fiscal measures, built environment regulation and school regulation. The article considers the types of law that the Commonwealth could pass under the trade and commerce, taxation, communications and corporations powers, along with the power to make tied grants to the States. It then considers how the States could pass such laws but avoid levying any duty of excise, restricting freedom of interstate trade and passing any law that would be inconsistent with a Commonwealth law.