Tonga's Crown Prince is will continue as the country's Minister of Foreign Affairs and take on the Defence portfolio in the newly-formed national government.

Prime Minister Lord Fatafehi Fakafanua, who won a parliamentary vote on December 15 to lead Tonga, announced the first of the ministerial roles for the cabinet just before Christmas, following last month's general election.

The Prime Minister's selections are waiting for approval from King Tupou VI in what is expected to be a formality.

Tupouto'a 'Ulukalala - the king's son and heir to the throne of the Kingdom of Tonga - was controversially appointed to head the Foreign Affairs portfolio in January, under outgoing Prime Minister Dr Aisake Eke's reign.

"The Crown Prince will continue as Minister for His Majesty's Armed Forces and Foreign Affairs," Lord Fakafanua confirmed in a statement.

The appointment of the Crown Prince, who is also married to Prime Minister's sister, to run defence will spark further concerns for advocates troubled by the king's growing influence.

The king also recently renamed the Tonga Defence Services His Majesty's Armed Forces.

According to the Tongan constitution, up to four ministers can be appointed to cabinet on the "advice" of King Tupou VI.

Lord Fakafanua is also one of nine traditional Tongan heredity nobles of the 26 members that can be appointed by the king to the Fale Alea's Legislative Assembly .

The other 17 sitting MPs are elected by the citizens of Tonga.

Prior to electoral reforms, only Tongan nobles were permitted to become prime ministers in a convention instituted in 1876 which lasted 130 years.

Since 2006 the overwhelming majority of democratically-elected representatives seemed to favour MPs that were not products of Tongan hierarchal chiefdoms.

That changed this month when - Tongan media reports - Lord Fakafanua grasped power in the lead-up to the parliamentary secret ballot after garnishing seven 'commoner' votes.

Lord Fakafanua won the prime ministerial ballot 16 votes to Dr Eke's 10 votes.

The Tongan electoral system, which has been undergoing further legislative reform since 2016, incorporates the last Polynesian monarchy.

The parliamentary system has a blend of traditional structures of chiefdom representation that has slowly merged with more democratic structures following rioting unrest that began on the streets of the capital in 2006.

Ministers like the Crown Prince appointed from outside general elections cannot vote in any vote-of-no-confidence against the Prime Minister.