Tonga is an archipelago directly south of Western Samoa. Its 169 islands, 36 of them inhabited, are divided into three main groups – Vava'u, Ha'apai, and Tongatapu – and cover an 800-kilometer (500 miles)-long north–south line. The largest island, Tongatapu, on which the capital city of Nuku'alofa is located, covers 257 square kilometers (99 sq mi). Geologically the Tongan islands are of two types: most have a limestone base formed from uplifted coral formations; others consist of limestone overlaying a volcanic base.

The climate is basically subtropical with a distinct warm period (December–April), during which the temperatures rise above 32 °C (90 °F), and a cooler period (May–November), with temperatures rarely rising above 27 °C (80 °F). The temperature increases from 23 °C to 27 °C (74 °F to 80 °F), and the annual rainfall is from 1700 to 2970 millimeters (67 to 117 in) as one moves from Tongatapu in the south to the more northerly islands closer to the Equator. The mean daily humidity is 80%.