Data collected from the Hikurangi subduction zone by an international team of scientists has shown the fault is far more active than previously thought.
The Hikurangi subduction zone, off the North Island's east coast, is where the Pacific plate subducts under the Australian plate.
It is New Zealand's largest and most active fault.
GNS Science, a research institute, now estimates that over the course of a year there could be thousands more earthquakes in the subduction zone than previously thought.
The majority are under magnitude 4.0 and have never been felt by people onshore, and never detected by onshore equipment.
The preliminary data findings come from a team of 17 scientists led by GNS, onboard Niwa's specialised research vessel Tangaroa, following the collection of earthquake-monitoring instruments from the east coast seabed.
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The instruments included a newly developed type of Ocean Bottom Seismometer built by Professor Spahr Webb and his team at Columbia University in New York.
He said the data quality from the new instrument was much higher than he expected, and that he was surprised by the large number of earthquakes they had recorded.
Project leader, GNS Science, Dr Laura Wallace said the Hikurangi subduction zone was potentially the largest source of earthquake and tsunami hazard in New Zealand, but there was still much to learn about it.
"Subduction zones are a type of fault and are responsible for the largest and most powerful earthquakes and tsunamis in the world, such as Sumatra 2004, Chile 2010 and Japan 2011."
Wallace said the initial data collected from the offshore instruments suggested more earthquakes were occurring than previously detected by onshore network of instruments operated by GeoNet.
"We really need instruments offshore the east coast of the North Island, continuously monitoring earthquakes and slow-slip events to get a complete understanding of all the creaks and groans of the Hikurangi subduction zone," Wallace said.
Slow-slip events were earthquakes which occurred over weeks to months, and were not felt on land as pressure was released slowly rather than suddenly in an earthquake, she said.
Scientists have deployed a further five instruments off the coast of Gisborne as part of this research voyage and will collect these instruments in a year's time.
All the data being collected will help scientists learn more about earthquakes and slow-slip events at the Hikurangi subduction zone, and what this means for future earthquakes in New Zealand.
This research is being undertaken under a five-year, $6 million MBIE-funded Endeavour project led by GNS Science, with major collaboration and contributions from other New Zealand and international partners.