The latest study of movements on the South Island's Alpine Fault has found new evidence that a devastating earthquake could strike any day.
Recent research shows the fault has ruptured four times in the past 900 years, most recently in about 1717, when an earthquake of at least eight on the Richter scale snapped trees halfway up their trunks and set off massive landslides throughout the Southern Alps.
Geologists have also found evidence of similar-sized or larger earthquakes from Alpine Fault movements in about 1620, 1450 and 1100.
Now, an investigation of the offshore part of the fault, south of Milford Sound, shows it is moving as fast as sections onshore and is just as likely to trigger a huge earthquake.
The study by the National Institute of Water and Atmospheric Research (Niwa) and the Institute of Geological and Nuclear Sciences (GNS) concludes the offshore fault is moving by as much as 3cm a year.
Niwa marine geologist Dr Phil Barnes said any sudden movement on this part of the fault could rupture the entire offshore section and propagate onshore, building the earthquake to magnitude 8.1 or higher.
Geologists believe the faultline moves between every 250 and 300 years. Given those figures, the chance of a major South Island earthquake increases every day, and may already be more than 30 years overdue.
The Alpine Fault is one of the world's most prominent geological features. Visible from space, it runs for about 600km up the spine of the island on the western edge of the Alps, marking the boundary of the Pacific and Australian Plates.
Scientists say the fault below ground level is moving about 3cm each year, but the surface fault is locked. Eventually, the tension between the two will snap like a rubber band and the surface fault will rupture, generating the biggest earthquake since the European settlement of New Zealand.
Any earthquake of more than 7.5 would cause severe ground shaking and building damage on the West Coast, in Christchurch, Nelson and Dunedin.
Road, rail, communication and electricity links across the South Island could be cut, huge landslides could dam rivers, cause flooding downstream and throw river control and hydro-electricity generation into chaos.
Centres largely built on soft sediments, like Christchurch, could experience soil liquefaction from the shaking, with buildings and other structures collapsing.
GNS natural hazards research manager Dr Kelvin Berryman cautioned that the findings did not necessarily mean the likelihood of a massive earthquake was higher than before, or that when it came it would be more intense than currently expected.
The offshore study used acoustic mapping technology to produce high resolution images of the fault's continuation south of Fiordland. Dr Barnes said the research would help make more accurate estimates of the frequency of large earthquakes on the fault.