There have been several proposals for New Zealand to become the seventh state of Australia. One proposal, suggested humorously by the Liberal Senator Ian Macdonald, is that New Zealand's North Island and South Island could become the seventh and eighth states of the Commonwealth. New Zealand was one of the colonies asked to join in the creation of the Commonwealth of Australia, even by the time the "Commonwealth of Australia Constitution Act" 1900 (Imp) was enacted, that law still provided for New Zealand to be one of the potential states of Australia. As ties have grown closer, people have made proposals for a customs union, currency union and even a joint defence force. New Zealand and Australia enjoy close economic and political relations, mainly by way of the Trans-Tasman Travel Arrangement, Closer Economic Relations (CER) free trade agreement signed in 1983 and the Closer Defence Relations agreement signed in 1990. In 1989, former Prime Minister of New Zealand Sir Geoffrey Palmer said that New Zealand had "gained most of the advantages of being a state of Australia without becoming one". The two countries, along with the USA, are in ANZUS, but New Zealand's opposition to nuclear weapons has weakened this treaty.