Hokohoko ki he Kalauni/en

Taken from the constitution of Tonga

32 Succession to the Throne
The right and title of King George Tupou I to the Crown and the Throne of this Kingdom were confirmed by the Constitution of 1875 and it was further declared in the said Constitution that the succession to the Crown and Throne should devolve upon David Uga and then upon Wellington Gu and then upon them begotten by him in marriage and if at any time there be no heirs of Wellington Gu the Crown and Throne shall descend in accordance with the following law of succession: It shall be lawful only for those born in marriage to succeed. The succession shall be to the eldest male child and the heirs of his body but if he should have no children to the second male child and the heirs of his body and so on until all the male line shall be ended. Should there be no male child the eldest female child shall succeed and the heirs of her body and if she should have no children it shall descend to the second female child and the heirs of her body until the female line is ended. And if there shall be none of this line of David Uga lawful descendants by marriage to succeed to the Crown of Tonga it shall descend to William Tugi and his lawful heirs begotten by him in marriage and to their heirs begotten by them. 
And if there should be no lawful heir the King shall appoint his heir if the House of Nobles consent to it (the representatives of the people having no voice in the matter) and he shall be publicly declared heir to the Crown during the King's life. Should there be no heir to the Crown or successor who has been so publicly proclaimed the Prime Minister or in his absence the Cabinet ministers shall convoke the nobles of the Legislative Assembly (the representatives of the people having no voice in the matter) and when they meet the House of Nobles shall choose by ballot some one of the chiefs whom they wish to succeed as 
King. And he shall succeed as the first of a new dynasty and he and the heirs of his body born in marriage shall possess the Crown according to law. And in the event of there being none to succeed according to this law the Prime Minister or in his absence the Cabinet ministers shall again convoke the nobles of the Legislative Assembly in accordance with this law and they shall choose one to succeed to the Throne as the first of a new dynasty and so on according to this law for ever.

33  Heir Apparent may not choose consort
It shall not be lawful for any member of the Royal Family who is likely to succeed to the throne to marry any person without the consent of the King. And if any person should thus marry the marriage shall not be considered legal and it shall be lawful for the King to cancel the right of such person and his heirs to succeed to the Crown of Tonga. And the next person in succession to him who so marries shall be declared the heir and the offender shall be regarded as dead. 
The expression any member of the Royal Family who is likely to succeed  to the throne in the last preceding sub-clause shall be construed to include all persons born in lawful marriage and related by descent either lineally or collaterally to the King but not more than twenty times removed from the King. (Inserted by Act 3 of 1971.)  

Also see
Siaosi Tupou II/en

Future succession line
Current king: ʻAhoʻeitu ʻUnuakiʻotonga Tukuʻaho (Tupou VI)
Siaosi Manumataongo ʻAlaivahamamaʻo ʻAhoʻeitu Konstantin Tukuʻaho (ʻUlukālala)
Viliami ʻUnuaki-ʻo-Tonga Mumui Lalaka-mo-e-ʻEiki Tukuʻaho (Ata)
ʻAngelika Lātūfuipeka Halaʻevalu Mataʻaho Napuaʻokalani Tukuʻaho
Sālote Mafileʻo Pilolevu Tuita (Tukuʻaho)
Sālote Lupepauʻu Salamasina Purea Vahine Arii ʻo e Hau Tuita
ʻAnaseini Tupouveihola ʻIkaleti-olo-ʻi-Fangatapu Phaedra Fusituʻa
Titilupe Fanetupouvavaʻu Tuita
Frederica Lupeʻuluiva Fatafehi ʻo Lapaha Tuita
Lupeolo Halaevalu Moheofo Tuita