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The quake hit near the island nations of Tonga and Fiji at a depth of 560km.
Its epicentre was 270km east of Levuka on Fiji's Ovalau Island and 443km west of Neiafu in Tonga, a town with a population of about 6000 people.
A Civil Defence and Emergency Management spokesman said there was no threat to New Zealand.
The quake was initially reported as a magnitude 8.0 and then upgraded to 8.2 - a magnitude that could cause tremendous damage had it not been so far down, the US Geological Survey said.
"People will feel it but it's so deep that I would not expect any damage," USGS geophysicist Jana Pursley told Reuters.
The US Pacific Tsunami Warning Centre said small tsunami waves have been observed.
"While no hazard is expected for any coastline, but please be observant and exercise normal caution."
The area is located on the earthquake-prone Ring of Fire and there have been several aftershocks of magnitude 5.4 today.
New Zealander Cara Michael, who is in Fiji with her family, said "buildings were swaying, made you feel a bit seasick almost, but it stopped after about half a minute".
Despite that everyone was "pretty relaxed here as you'd expect", she told The New Zealand Herald.
One person in Lautoka, the second biggest city in Fiji, said they felt the whole building shake, making them think it was flexible.
No casualties were expected based on initial readings and there have been no reports of damages.
The quake was not on the fault line where the Pacific Plate and the Indo-Australian Plate meet. The plates runs almost the entire length of New Zealand's South Island.
- NZ Herald and Reuters