Tonga's Crown Prince Tupouto'a 'Ulukalala has been named minister for defence and foreign affairs, taking on the key government roles in the Pacific Islands nation a week after prime minister 'Aisake Eke was appointed by King Tupou VI.

Tonga, with a population of 107,000, is one of the few South Pacific nations with its own military, and the only Pacific nation with a sovereign indigenous monarchy.

Eke takes on three additional ministerial roles - for finance, fisheries and prisons - in the newly formed 11-member cabinet, Tonga's government said in a statement on Tuesday.

Crown Prince Tupouto'a 'Ulukalala, 39, previously studied at Australia's defence college in Canberra.

Tonga's parliament has 17 representatives who are elected by the population and nine who are chosen by the country's nobility.