For example , the main island in the Kingdom of Tonga , where the capital Nuku ` alofa is located and most of the population resides , is called `` Tongatapu '' . In the main island of the Kingdom of Tonga , where the greater portion of the population reside within the capital Nuku ` alofa , the word is often appended to the end of `` Tonga '' , making the word `` Tongatapu '' , as `` Sacred South '' rather than `` forbidden south '' .