The South Fiji Basin ( also Fiji Basin ) is a large 4 to 4.7 km (2.5 to 2.9 mi) deep oceanic basin in the south-west Pacific Ocean, south of Fiji. It was formed from the then Indo-Australian plate and is delimited to the north west by the New Hebrides Trench, and the Hunter fracture zone, to the west by the Three Kings Ridge, to the east by the Lau-Colville Ridge, and to the south by the continental shelf of Zealandia.
The Lau–Colville Ridge is an extinct oceanic ridge located on the oceanic Australian Plate in the south-west Pacific Ocean extending about 2,700 km (1,700 mi) from the south east of Fiji to the continental shelf margin of the North Island of New Zealand. It was an historic subduction boundary between the Australian Plate and the Pacific Plate and has important tectonic relationships to its east where very active spreading and subduction processes exist today. It is now the inactive part of an eastward-migrating, 100 million year old Lau-Tonga-Havre-Kermadec arc/back-arc system or complex and is important in understanding submarine arc volcanism because of these relationships. To its west is the South Fiji Basin whose northern bedrock is Oligocene in origin.