Monday's devastating earthquake has been upgraded from 7.5 to 7.8 magnitude.
GNS Science after reassessing the data from its stations across the country.
The change means aftershocks may be larger in magnitude for a longer period of time.
"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," Acting Civil Defence Minister Gerry Brownlee 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," he said.
"It simply provides us with more knowledge about how significant this earthquake was."
"As expected with a larger earthquake, the revised magnitude does have an effect on the probabilities of forecast aftershocks, meaning it is now forecast that aftershocks 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," Brownlee said.
Meanwhile, the HMNZS Canterbury will this evening set sail to Lyttelton with about 390 evacuees from Kaikoura on board.
Brownlee said ship is expected to arrive around 11pm, depending on weather conditions and the amount of time it takes to upload those waiting in Kaikoura.
"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," he said.
"Meanwhile, the 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 said.
New Zealand's biggest earthquakes
8.2 - Wairarapa, January 23, 1855
7.8 - North Canterbury, November 14, 2016
7.8 - Murchison, June 17, 1929
7.8 - Hawke's Bay, February 3, 1931
7.8 - Dusky Sound, July 15, 2009
7.6 - Pahiatua, March 5, 1934
7.5 - Marlborough, October 16, 1848
7.5 - Hawke's Bay, February 23, 1863
7.5 - Cape Farewell, October 19, 1868
7.3 - Hawke's Bay, February 13, 1931
7.3 - Auckland Islands, December 30, 2007
7.3 - North Canterbury, September 1, 1888
7.2 - Puysegur Trench, south-west of Fiordland, November 23, 2004
7.2 - Wairarapa, June 24, 1942
7.1- Darfield, September 4, 2010
7.1 - Fiordland, August 22, 2003
7.1 - Arthurs Pass, March 9, 1929
7.1 - Inangahua, West Coast, May 24, 1968
7 - East Cape, February 6, 1995
7 - Wairarapa, August 2, 1942
6.9 - Nelson, February 12, 1893
6.8 - Gisborne, December 20, 2007
6.6 - Grassmere, August 16, 2013
6.5 - Edgecumbe, March 2, 1987
6.5 - Cook Strait, July 21, 2013
6.3 - Christchurch, February 22, 2011
6 - New Brighton, December 23, 2011
6 - Christchurch, June 13, 2011
Source: GNS Science