© GeoNetA silent earthquake's movement is plotted
GeoNet said today that the event, which could move faults at the equivalent of magnitude 5 or higher regular earthquakes, had just been detected and was being monitored.
The slow-motion earthquakes, also known as "slow slips", are undetectable by humans or seismographs, and are instead measured using changes in distance between global positioning system stations across the North Island.
They have been shown to be able to trigger - or alleviate - large, tsunami-generating earthquakes.
Follow slow-slip motion on GeoNet
The agency said a magnitude 4 earthquake off the coast of Gisborne last week was likely related.
The phenomenon is fairly new to science and, after being discovered in the United States, was first located in New Zealand in the early 2000s.
GNS Science seismologist Stephen Bannister said in 2014 the earthquakes occurred every one to two years in the Gisborne area, pushing sections of Poverty Bay eastward by 2-3cm.
They are found around the world in subduction margins, where one tectonic plate is slipping under another into the earth's mantle.
New Zealand scientists and colleagues from the US and Japan have been studying silent earthquakes in the Gisborne and Hawke's Bay areas since May 2014, comparing the timing of earthquakes with the motion of the sea floor.
The group, known as HOBITTS (Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip), was hoping its research could improve scientists' ability to predict earthquake hazards.
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