In November 1990 Australia extended its territorial sea from 3 to 12 nautical miles. This article examines the consequences of this extension under international and municipal law, and draws comparisons with the experience of the United States and Canada in relation to their territorial seas. The expansion of Australia's territorial sea has some noteworthy features under international law in its effect on Australia's territorial claims in the Antarctic, and on the maritime delimitation between Australia and Papua New Guinea in Torres Strait. The consequences of the extension under municipal law arise from the unique offshore regime agreed between the federal government and seven state and territorial governments in 1979, by which jurisdiction over the territorial sea is divided between central and regional governments. Australia's federal constitutional structure has created problems of offshore jurisdiction similar to those experienced in Canada and the United States, but the solution adopted is markedly different. The Australian settlement may prove a useful model for federations trying to reach an agreement over offshore areas.