The interval between ruptures of New Zealand's major fault line running through the Southern Alps is shorter than previously thought, according to a major new study.

The Alpine Fault, along the spine of the South Island, is assessed by scientists to be likely to produce a large earthquake in coming decades, a potentially catastrophic event that would change the face of the country.

On average, quakes of magnitude 7.5 or larger had previously been thought to strike along the Alpine Fault every 329 years, plus or minus 26 years.

The last big event, measuring about magnitude 8.0, occurred exactly 300 years ago.

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But now, scientists have revised the average time between large quakes to one every 291 years, plus or minus 23 years.

There is a 29 per cent chance of a big "surface rupturing" quake within the next 50 years, according to a new study by GNS Science that made use of optical fibre equipment drilled into the Alpine Fault.

The scientists looked at sediment evidence from John O'Groats wetland near Milford Sound and Hokuri Creek, 20km north of where they found a record of 22 major quakes over a 7000-year period.

"The preferred John O'Groats-Hokuri Creek earthquake record consists of 27 events since 6000BC for which we calculate a mean recurrence interval of 291 plus 23 years, shorter than previously estimated for the South Westland section of the fault and shorter than the current interseismic period," says the study, published in Earth and Planetary Science Letters.

GNS Science maintains a major Alpine Fault rupture would produce one of the biggest earthquakes since European settlement of New Zealand and have a "major impact" on the lives of people across the country.