New Zealand's largest faultline is more active than scientists previously thought and could generate "megathrust" earthquakes up to magnitude 9.0, and giant tsunamis.

A year after the magnitude-7.8 2016 Kaikoura earthquake, scientists fear that it wasn't "the big one".

The Hikurangi fault subduction zone, which spans offshore from the east of Gisborne to around Kaikoura and the top of the South Island, had been thought dormant before the massive Kaikoura tremor, Fairfax reports.

A quake on the Hikurangi subduction zone could prove even more devastating than the magnitude 7.8 that destroyed houses, lifted the Kaikoura seabed by 2m, tore apart farmland, and wrecked kilometres of State Highway 1, GNS scientist Ursula Cochran said.

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"We need to think Japan 2011 basically, because if our whole plate boundary ruptured, it would be a magnitude 9 earthquake," Cochran told Fairfax.

"One thing about reflecting on from the Kaikoura earthquake is we don't want people to think this is the big one."

An expedition of 30 scientists, jointly led by Niwa and the University of Auckland, is spending six weeks at sea off the coast of Gisborne to study the Hikurangi subduction zone.

Part of the Pacific "ring of fire" where the Pacific tectonic plate dives beneath the Australian plate, the scientists will look at slow-slip events and submarine landslides to help reveal their causes and improve understanding of the risk that the plate boundary poses to communities along the East Coast.

The slow slip events in the northern part of the Hikurangi subduction zone happen in relatively shallow depths beneath the seafloor where data can be collected to help reveal how they work.

"Right now, we can only speculate about what's driving them, for a slow slip event to occur you require the fault to move a little and slowly, without the movement advancing into a normal earthquake rupture," AAP reported expedition co-leader marine geologist, Dr Philip Barnes said.

"Since their discovery scientists have come to understand that there is a whole continuum of seismological processes between constant creep on a fault and big earthquakes, and slow slips are one of those."

Last year's Kaikoura earthquake triggered a large slow slip event offshore the East Coast covering an area of more than 15,000sq km.

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Tsunami modelling shows that a magnitude-9 rupture on the Hikurangi subduction zone means waves would hit parts of the south Wairarapa coast in just seven minutes, and between 10 to 30 minutes to reach Marlborough's Cloudy Bay, Fairfax reported Cochran as saying.

Quakes on subduction zones caused widespread devastation in Sumatra in 2004, Chile in 2010 and Japan a year later.

While the magnitude-7.8 Kaikoura event released 177 times as much energy as the mag-6.3 February 2011 Christchurch quake, a mag-9.0 megathrust would be more than 11,000 times as powerful.