The Tonga archipelago includes 170 islands divided into 4 separate groups. Starting with Niuas in the N, working S through Vava'u, Ha'apai and finally Tongatapu, at the southern end of the archipelago. Lying about 3,000km (1,875mi) to the E of Australia, Tongatapu is Tonga's main island, including the capital Nuku'alofa. Tongatapu is made up of a raised coral platform, where constant wave action has cut a shelf into the cliff bound south coast, but to the north there are some low lying reefs with many offshore islets. Most surf spots are all squeezed onto a remarkable reef bend on the W side of the island, where trade winds blow straight offshore. SW swells produce grinding lefts through the S hemisphere winter, (April-Oct), whilst less consistent N swells favour the shorter, shallow rights from Nov-March.