Tongatapu Group

Tonga is a constitutional monarchy of 169 islands scattered across 800 km of South Pacific from Tongatapu in the south to the Niuas in the far north. The tidal regime across the kingdom is mixed semidiurnal: two highs and two lows per day with substantial inequality between them. Spring range at Nuku'alofa reaches 1.3–1.5 m above Chart Datum; the mixed character means the two daily highs can differ by 0.4 m or more — tidal planning that assumes two equal cycles will sometimes be caught out. Tongatapu is a flat limestone island at 1–3 m elevation, acutely sensitive to any combination of high spring tide and storm surge. The south coast faces a sheltered reef lagoon; the north coast is exposed to ocean swell. Nuku'alofa harbour occupies the northwest corner of the island. The Royal Palace, built in 1867, stands on the harbour foreshore. Ferry departures to Vava'u, Ha'apai, and the Niuas all leave from Nuku'alofa wharf, where tidal state governs safe departure windows for deep-draught inter-island vessels. Vava'u, 250 km north, centres on the Port of Refuge — a drowned volcanic crater system creating 40 km of navigable, fully enclosed, reef-protected water. The settlement of Neiafu sits above the inner harbour. July through October is humpback whale season: whales migrate north from the Southern Ocean to calve and mate, and Vava'u is among the world's premier sites for permitted in-water encounters with licensed operators. Ha'apai, centred on Lifuka island and Pangai town, is the most exposed and isolated of the three main groups. These low coral atolls sit barely 5 m above Chart Datum at their highest points, carrying the full weight of open Pacific swell on their windward faces. Wave-driven water-level variation often exceeds the tidal signal on the outer shores. The reward for the isolation is exceptional coral cover, empty beaches, and fishing grounds that see only a handful of visiting boats per season.