Complex sentence: The southern half was colonised in the same year by the United Kingdom as British New Guinea, but in 1904 with the passage of the Papua Act, 1905, was transferred to the newly formed Commonwealth of Australia which took on its administration.
Split into simpler sentences: The southern half was colonised in the same year by the United Kingdom as British New Guinea. In 1904 with the passage of the Papua Act, 1905, it transferred this territory to the newly formed Commonwealth of Australia, which took on its administration.