The foundational legal document of the Australian federation, which came into effect on 1 January 1901 when the six colonies united to form the Commonwealth of Australia. The Constitution establishes the structure of the federal parliament, the executive government, and the judiciary, and sets out the division of legislative powers between the Commonwealth and the states. Under this division, the Commonwealth parliament has exclusive power over certain matters (such as defence, currency, and customs), concurrent power over others that the states may also legislate on (such as taxation and education), and residual powers — everything not specifically mentioned — remain with the states. Where a valid Commonwealth law conflicts with a state law on a concurrent matter, the Commonwealth law prevails. The Constitution can only be changed by referendum, requiring approval from a national majority of voters and a majority of voters in at least four of the six states. It is interpreted and enforced by the High Court of Australia, which serves as the final arbiter of constitutional disputes between the Commonwealth and the states.