However, GNS later revised the magnitude down and updated its location to 30km west of Arthur's Pass - in the West Coast region.
While more than 3000 people from Mandeville in Southland to the central North Island reported feeling the quake, only about six insurance claims had so far been lodged with the Earthquake Commission, a spokeswoman said.
"But we're keeping a watch on the situation and if people think they've suffered damage they can lodge a claim on 0800 DAMAGE or go to the EQC website."
All claims had come from the Canterbury region, she said.
The tremor prompted roading and rail companies to assess for damage to the country's infrastructure.
The New Zealand Transport Agency said it had been checking all state highways in Canterbury and the West Coast for any damage from the tremors but there was none.
KiwiRail closed some of its South Island rail lines for a safety inspection but no issues were reported and a safety clearance had been given to reopen most of them.
"We are awaiting final clearance on the Stillwater and Midland lines," a spokeswoman said.
GNS seismologist John Ristau said it was not unusual to have earthquakes of this size in the area.
"We've had them in the past century, there have been at least a couple of others that have been above magnitude 6."
Aftershocks would go on for a while yet, which could reach the magnitude 5 range, he said.
"Just be aware there could be a sizeable aftershock coming - but beyond that we can't say for certain whether there'll be anything bigger coming."
Sam Hay, who works at the Otira Hotel, not far from the quake's epicentre, said he was woken by a "pretty decent shake".
But having experienced the major Christchurch earthquakes, he was not rattled enough to get out of bed.
There was no damage to the hotel, he said.
"It was a decent shake, but it wasn't a violent shaking like we got in Christchurch - it was a gentle 'I'm on a boat in the ocean' shaking."
Debbie Stewart from the Arthur's Pass Cafe and Store also said she felt a "jolt", but nothing came off the shelves.
"I thought more of Christchurch really and wondered if we got that, what did they get?"
Quake felt around the country
The quake was felt strongly in Hokitika and Greymouth, and sent some people scurrying for cover.
A long-time Arthur's Pass bach owner whose chimney had fallen through the roof, John Charles, told Radio New Zealand he was on his way to collect the paper when the earth started to move.
"It was quite strange, normally you get a sense that something's going to happen. But it just struck out of the blue. It was really violent," Mr Charles said.
Quakes in the alpine village usually ended in a "snap", which was what sent things tumbling off the shelves.
"But it just stopped. It lasted about 30 seconds, the house was creaking and lurching. Stuff on the steel bench was vibrating."
The aftershocks were far gentler, unlike in past earthquakes, he said.
"There are slight sensations, like a ripple on a lake."
Mr Charles was asked to check on some baches, and also walked to the town store, but saw no damage. The goods in the store were still on the shelves, he said.
Nearby resident Cody Peneamene was awoken by the first earthquake.
"We felt the first handful. You could hear them - it sounded very similar to when the coal train rumbles through," Mr Peneamene said.
Solid Energy's coal transport schedule was disrupted by the earthquake, but not by enough to make the company miss a shipment.
The disruptions came as KiwiRail suspended rail lines to inspect them for damage. The state energy company uses the Arthur's Pass line and the Midland line to transport coal from the West Coast to Lyttelton for export.
Solid Energy spokesman Bryn Somerville said that overall, the company was "one train down" but due to flexibility in the system, it would not impact on shipments.
The rail lines would have to be closed for a lot longer in order to have a significant impact.
Westland Milk Products was unaffected by the suspension of the rail line through Arthur's Pass, which it uses to transport milk to and from Canterbury.
The TranzAlpine passenger train was suspended, and a bus alternative was arranged for passengers.
Department of Conservation staff in Arthur's Pass said there had been no reports of damage to tracks or other DOC recreational facilities in the area.
Arthur's Pass, so close to the Alpine Fault, is no stranger to earthquake. A 7.1 magnitude quake struck in 1929, reportedly shaking the village for a horrifying four minutes, and a 6.7 struck in 1994.
What to do during an earthquake:
* If inside a building, move no more than a few steps, drop, cover and hold. Stay indoors until the shaking stops and you are sure it is safe to exit
* If inside an elevator, drop, cover and hold. When the shaking stops, try to get out at the nearest floor if you can safely do so
* If outside, move no more than a few steps away from buildings, trees, streetlights, and power lines, then drop, cover and hold
* If at the beach or near the coast, drop, cover and hold, then move to higher ground immediately in case a tsunami follows the quake
* If driving, pull over to a clear location, stop and stay there with your seatbelt fastened until the shaking stops. Once the shaking stops, proceed with caution and avoid bridges or ramps that might have been damaged
* If in a mountainous area or near unstable slopes or cliffs, be alert for falling debris or landslides
Source: http://www.getthru.govt.nz/disasters/earthquake
- Additional reporting: Laura Mills of the Greymouth Star and Aimee Brill of the Westport News
