<>This is an extract from the first chapter of the history book I am writing.
In early 1855 there was an earthquake in the centre of Cook Strait, thought to be of a magnitude of about 8.1-8.2, the largest in human memory.  (Maori elders said there was no record in their traditions of an earlier shock which was as great.) There have been a few subsequent earthquakes of just under magnitude 8 including the Hawke’s Bay earthquake of magnitude 7.8; there were 258 fatalities. The 2010 Glendale earthquake was magnitude 7.1 and the largest 2011 Christchurch aftershock was magnitude 6.3 with 185 deaths.  One additional unit of magnitude represents about 30 times the energy.
The violence of the 1855 shock was recorded by the small and scattered population, especially in Wellington where it had a major impact, although it was severely felt throughout the bottom half of the North Island and the top half of the South Island. It was a consequence of the westward descent of the Pacific Plate beneath the southern part of the North Island (the eastern edge of the Australian Plate).
The earthquake fault or fissure in the earth running north east from the middle of Cook Strait through the Wairarapa was about 140kms long, while the aftershocks lasted for about nine months (although there was no systematic seismic measurement in those days; the first seismograph arrived in 1880). The quake was of international interest to the extent that the eminent geologist Charles Lyell seized on it as illustrating his thesis that the earth moved incrementally along geological fault lines during earthquakes. (The popular explanation for the 1855 earthquake was an eruption from an underwater volcano.)
Some of the ground shifted 12 metres horizontally along the fault, while on the western (Rimutaka Range) side of the fault, the land was lifted by as much as 6.5 metres. Earlier earthquakes in the region are identifiable at Turakirae Head marking the western headland of Palliser Bay, where the Wairarapa Fault runs offshore into Cook Strait, by three stranded gravel beaches between strips of rocky ground. The second to lowest (i.e. the beach uplifted during the 1855 earthquake) is about 9 metres higher (that is, 16 metres above sea level) occurred about 250 BCE (2200 BP). The next stranded beach, another metres higher, seems to have been uplifted about 2850 BCE (4800 BP), with an even earlier uplift of 7 metres in about 4850 BCE (6800 BP). These radio carbon ages (from shells and woods) are subject to a margin of error of plus or minus several hundred years, and there also seems to have been two large earthquakes which impacted in the Wairarapa in about 1050 CE and 1450 CE (900 and 3400 BP) which did not cause any recognisable uplift at Turakirae Head. Collectively the geological evidence suggests there have been earthquakes big enough to cause surface rupturing along the Wairarapa Fault every 800 to 2100 years. (This does not mean Wellington has no significant earthquake worries in the next millennium. There are other major fault lines.)
There were enough humans recording what happened in 1855 to get a sense of the effect on the biota. A tsunami swept around the coast including inundating the Rongotai isthmus (Kilbirnie) into Evans Bay. The wave was almost nine metres high in Palliser Bay. There was considerable devastation to plant life, and extensive landslips (especially from the Rimutaka Range) but none fatal. Bottom living fish (notably ling) suffered rapid vertical movements due to the submarine slumping in Cook Strait and floated dead in Cook Strait; inshore fish were swept onto the land by the tsunami. One can’t help wondering whether that explains part of the substantial depletion of the rich fishing stock which the first Wellington settlers came across (in which case subsequent fishing practices reduced the recovery).
Some of the shellfish stranded by the coastal uplift had no chance. Certainly there is little evidence today of the rock oysters that were common around Wellington before the earthquake. (However Manukau Harbour has also suffered severe depletion despite there being no earthquake, suggesting both stocks may have been eaten out.) More illustrative a rock-boring mollusc (Anchomasa Stimulus ) is now very rare; they certainly were never eaten by humans.
There was widespread destruction of human supplied physical capital especially brick chimneys but it unclear to what extent that was because of their flimsy construction. Settlement was only fifteen odd years old and earthquake robustness was a still evolving consideration. (A cluster of strong earthquakes in 1848 had also led to widespread building damage.)
Estimates of how many humans died from the earthquake are put between 5 and 9; one settler in Wellington, four to six in Maori in the Wairarapa and possibly two others in the Manawatu, a rate of between 5 and 9 per 100,000 (for the population across the whole country). In contrast the 256 deaths of the 1931 Napier earthquake were 17 per 100,000 and the 185 deaths from the February 2011 Canterbury earthquake was 4 per 100,000.
 This section is based on R. Grapes (2000) Magnitude Eight Plus: New Zealand’s Biggest Earthquake, with subsequent updatings by the author, pers. com. The Visitation. Rodney Grapes subsequently kindly updated the study for me. (The earlier 7.5 1848 Marlborough Earthquake is described in R. Grapes (2011).)
 A useful comparative source is http://www.mch.govt.nz/perspectives/earthquakes/.