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Norfolk Island was originally a colony acquired by settlement but was never within the British Settlements Act. It was accepted as a territory of Australia, separate from any state, by the Norfolk Island Act 1913 (Cth), passed under the territories power (Constitution section 122) and made effective in 1914. In 1976 the High Court of Australia held unanimously that Norfolk Island is a part of the Commonwealth. Again, in 2007 the High Court of Australia affirmed the validity of legislation that made Australian citizenship a necessary qualification for voting for, and standing for election to, the Legislative Assembly of Norfolk Island.

Who decided in 1976 that Norfolk Island is a part of the Commonwealth?
High Court of Australia