People have been evacuated from Kaikoura by the NZ Defence Force. The major earthquake that struck on Monday has been revised up from magnitude 7.5 to 7.8.

The magnitude of the earthquake on Monday has been revised from a 7.5 to 7.8 by GNS Science.

The earthquake, which struck just after midnight, was centred 15km north-east of Culverden, near Hanmer Springs, at a depth of 15km.

However, GNS scientist Kelvin Berryman said the upgrade of the quake did not change the level of the response.

Iain McGregor/Fairfax NZ People around the country are dealing the with aftermath of the powerful tremor that lasted for a long time.

"A simple way of saying it is actually, we've already been dealing with a 7.8 earthquake, we've just been calling it 7.5," he said.

But it did have an affect on the modelling of aftershocks.

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"Of course a larger magnitude earthquake means probably a larger and longer duration of aftershocks than we had signalled originally."

Lawrence Smith/Fairfax NZ Residents of Kaikoura in north Canterbury collect water in the centre of town on Wednesday. The magnitude of the quake has today been upgraded to 7.8.

New data showed that every day, for the next 20 days, New Zealanders could expect an aftershock of magnitude 5 or greater was likely to occur.

The aftershocks had however, remained contained within what was considered a normal area, he said.

The reason it had taken a number of days to discover the full magnitude of the quake was due in part to the size and complexity of it.

Stacy Squires The earthquake caused "utter devastation" in North Canterbury.

The Geonet system also required a very quick production of data and information, but the complexities of very large quakes could take some time to unravel.

"Those seismic waves recorded up to 500km away from the epicentre, are normally needed to be included in the magnitude calculation when we get to these very large magnitudes.

"It was also signalled by the fact that it took more than one minute for that fault to rupture from one end to the other, so there's a very long rupture time.

"We've also have significant new data coming in from the GPS network around the country about where movements have taken place."

Acting Civil Defence Minister Gerry Brownlee said GNS Science reassessed data from stations across the country.

"Because it took over a minute for the fault to rupture during this event, the standard method normally used to calculate the energy released during an earthquake was insufficient," he said.

"The revised magnitude just tells us what anyone who felt the earthquake would already know – that it was a powerful tremor that lasted for a long time.

"It does not change what happened or how central government or local authorities responded. It simply provides us with more knowledge about how significant this earthquake was."

Brownlee said the revised magnitude does have an effect on the probabilities of forecast aftershocks, meaning they may be larger in magnitude for a longer period of time.

"We know from the Christchurch earthquakes, the likelihood of earthquakes greater than magnitude 5 decreases over time."

He added that the HMNZS Canterbury will set sail to Lyttleton, Christchurch with about 390 evacuees from Kaikoura on board.

"The ship is expected to arrive around 11pm tonight, depending on weather conditions and the amount of time it takes to upload those waiting in Kaikoura," he said.

"The New Zealand Defence Force has now evacuated about 600 people from Kaikoura. Evacuations will continue for as long as necessary and, as of tomorrow, several ships from other nations – including the USS Sampson – will arrive off the Kaikoura coast ready to be mobilised as required.

NZDF is continuing to inspect the inland route into Kaikoura from the south, through Waiau, to determine the condition of the road.

"At this stage, it's hoped the route will be cleared by the weekend. NZDF is using all-terrain vehicles on the road and, once reopened, access will be controlled by the New Zealand Police," Brownlee says.