Tofua is a volcanic island in Tonga. Located in the Haʻapai island group, it is a steep-sided composite cone with a summit caldera. It is part of the highly active Kermadec-Tonga subduction zone and its associated volcanic arc, which extends from New Zealand north-northeast to Fiji, and is formed by the subduction of the Pacific Plate under the Indo-Australian Plate. It lies about 100 km (62 mi) above a very active seismic zone. It is connected to the nearby island of Kao by a submarine ridge.

Haʻapai is a group of islands, islets, reefs, and shoals in the central part of Tonga. It has a combined land area of 109.30 square kilometres (42.20 sq mi). The Tongatapu island group lies to its south, and the Vavaʻu group lies to its north. Seventeen of the Haʻapai islands are inhabited. Their combined population is 5,419. The highest point in the Ha‘apai group, and in all of Tonga, is on Kao, which rises almost 1,050 metres (3,440 ft) above sea level.