Lau-Colville Ridge in the context of "South Fiji Basin"

Zealandia (pronounced /ziːˈlændiə/), also known as Te Riu-a-Māui (Māori) or Tasmantis (from Tasman Sea), is an almost entirely submerged mass of continental crust in Oceania that subsided after breaking away from Gondwana 83–79 million years ago. It has been described variously as a submerged continent, continental fragment, and microcontinent. The name and concept for Zealandia was proposed by Bruce Luyendyk in 1995, and satellite imagery shows it to be almost the size of Australia. A 2021 study suggests Zealandia is over a billion years old, about twice as old as geologists previously thought.

By approximately 23 million years ago, the landmass may have been completely submerged. Today, most of the landmass (94%) remains submerged beneath the Pacific Ocean. New Zealand is the largest part of Zealandia that is above sea level, followed by New Caledonia.

The Tonga–Kermadec Ridge is an oceanic ridge in the south-west Pacific Ocean underlying the Tonga–Kermadec island arc.It is a result of the most linear, fastest converging, and seismically active subduction boundary on Earth, the Kermadec–Tonga subduction zone, and consequently has the highest density of submarine volcanoes.

The Tonga–Kermadec Ridge stretches more than 3,000 km (1,900 mi) north-northeast from New Zealand's North Island. The Pacific Plate subducts westward beneath the Australian Plate along the ridge. It is divided into two segments, the northern Tonga Ridge and southern Kermadec Ridge, by the Louisville Seamount Chain. On its western side, the ridge is flanked by two back-arc basins, the Lau Basin and Havre Trough, that began opening at 6 Ma and 2 Ma respectively. Beyound the basins is the Lau-Colville Ridge. Together with these seafloor structures the ridge forms the eastward-migrating, 100  million year old Lau–Tonga–Havre–Kermadec arc/back-arc system or complex.

The Lau Basin is a back-arc basin (also addressed as "interarc basin") at the Australian-Pacific plate boundary. It is formed by the Pacific plate subducting under the Australian plate. The Tonga-Kermadec Ridge, a frontal arc, and the Lau-Colville Ridge, a remnant arc, sit to the eastern and western sides of the basin, respectively. The basin has a raised transition area to the south where it joins the Havre Trough.

Ono-i-Lau is a group of islands within a barrier reef system in the Fijian archipelago of Lau Islands (ono means "six" in the Fijian language). There are four central volcanic islands: Onolevu, Doi (or Ndoi) Lovoni and Ndavura, the uppermost parts of the volcanic edifice rising from the Lau Ridge slightly more than 1,000 metres (3,300 feet) below sea level and on which the reef and other islands are built. There are also three clusters of coral limestone islets, Yanuya (50 islets) and Mana (46 islets) on the barrier reef and Niuta (7 tiny islets). A sand cay, Udui, is not counted as one of the six “islands”.

The group forms one of the southernmost of the Lau Islands; it is located at 20.80° South and 178.75° West, and occupies an area of 7.9 square kilometres (3.1 square miles). It has a maximum elevation of 113 metres (371 feet). It is 90 kilometres (56 mi) south-southwest of Vatoa, the nearest largish island; the only land further south in the Lau Group is in the two small Tuvana islands of coral limestone and sand.

The Havre Trough (Havre Basin) is a currently actively rifting back-arc basin about 100 km (62 mi) to 120 km (75 mi) wide, between the Australian Plate and Kermadec microplate. The trough extends northward from New Zealand's offshore Taupō Volcanic Zone commencing at Zealandia's continental shelf margin and continuing as a tectonic feature, as the Lau Basin which currently contains active seafloor spreading centers. Its eastern margin is defined by the Kermadec Ridge created by Pacific Plate subduction under the Kermadec microplate, while the western margin is the remnant Lau-Colville Ridge.