The Pacific Tsunami Warning Centre has cancelled its tsunami warning for New Zealand, while Civil Defence authorities said only a small wave was generated by tonight's big Fiordland earthquake.
GNS Science upgraded its reading tonight for the quake to 7.8 magnitude, bringing it into line with international readings.
It struck at 9.22pm, centred in Fiordland, and was felt widely throughout the South Island and as far north as Taranaki. No major damage was reported, though there were some power outages in the south.
An aftershock measuring 6.1 hit 20 minutes later.
An emergency management spokesman in Wellington, Vince Cholewa, said initial reports indicated a wave - around the 17cm height predicted by the Hawaii warning centre - arrived about 10:30pm in Bluff. "We're just waiting on confirmation of that, and we will cancel the warning for a potential tsunami," he said.
The Hawaii centre cancelled its alert just before 11pm.
GNS Science initially graded the quake as 6.6, contrasting with an estimate of 7.8 by the Pacific tsunami warning centre in Hawaii, but later upgraded it.
The quake was followed by a aftershock of 6.1 magnitude 20 minutes later.
Because of the size of the quake, and an initial reading in Hawaii of an 8.2 magnitude, Civil Defence issued a precautionary tsunami alert for Southland. The Southland Civil Defence Emergency Management Group activated the region's emergency operations centre to assess damage reports.
Group Controller Neil Cruickshank said that the emergency operations centre was monitoring the situation in Bluff, through people and recording equipment based there.
It appeared the tide was out and a destructive tsunami was "very unlikely," he said.
"We urge people to stay away from the immediate foreshore as a precaution. Our advice is that people do not need to evacuate their homes."
Lord Howe Island, off the coast of Australia, is under high alert, and locals in low-lying areas are moving to higher ground.
Tsunamis increase in size dramatically as they approach the coastline, and the first wave might not necessarily be the largest.
An Invercargill man told NZPA the initial quake lasted at least a minute.
"Things just started to rattle a bit, then the house started to sway."
He and his wife got their three young children out of bed and huddled under the dining table to wait it out. Cracks had appeared around several door frames, he said.
Central Southland man Warren MacPherson said a hanging light in his house would have been swaying "a good six inches each way".
He was on the phone when the quake struck and rushed outside.
"By geez, there was a fair bit of movement," he said.
Invercargill police Inspector Olaf Jensen said there were no immediate reports of damage in the southern city, but the quake was significant enough to send staff into doorways.
He described it was a strong, rolling quake rather than a sharp jolt.
One of New Zealand's biggest quakes - a magnitude 7.1 tremor - hit Fiordland near Secretary Island off the Fiordland coast on August 22, 2003. It caused significant landslides in parts of the region.
Scientists recorded about 5000 aftershocks over several months in the wake of the 2003 quake.
Aftershocks occur as the earth's crust adjusts to stresses caused by the main shock, and no two aftershock sequences are exactly the same.
The quake struck at a depth of 12km. It could be felt as far away as Wellington and Taranaki in the North Island, according to emails and calls from 3 News viewers.
Civil Defence says: "It is possible that there is a tsunami risk to New Zealand from the 7.8 earthquake that occurred near Tuatapere today." It says it is continuing to monitor the situation.
Emails and calls to 3 News have described the quake as large and long-lasting, but causing minimal damage. Power has been knocked out in Queenstown and Southland. A water main in Winton has reportedly been knocked out. The extent of damage is otherwise still unknown. No casualties are known at this stage.
Wendy London write to 3 News: "I felt the earthquake – and a second one or after shock, in Wellington.I’m on the 6th floor of a 2 year old apartment block just off Tory Street, and it was a barely perceptible rolling motion."
Mareike Hachemer writes from Dunedin: "Feels as if the house was a boat. Everything is shaking slightly. Lamps and curtain pieces are shaking."
And in Mosgiel, Blake McFarlane writes: "We were in the local United Video and the advertising signs on the roof were swaying something shocking, this was the best earthquake I've ever felt, was real rolling like."
It is believed to be the largest tremor in New Zealand since the 1931 Hawke's Bay quake.
3 News / NZPA
Listen to Southland Controller for Civil Defence Neil Cruikshank talk to Marcus Lush on RadioLIVE.