Although Tonga has been a constitutional monarchy since 1875, under treaties of friendship of 1879 and 1900 the kingdom ceded control of its foreign affairs to Great Britain represented by the British Agent and Consul who reported to the High Commissioner for the Western Pacific. This situation remained until 1970 when Tonga became fully independent.
From the description of BCT 2 [manuscript] : Records. 1954-1970. (National Library of New Zealand - Wellington Service Centre). WorldCat record id: 228114654