After five years of being stuck to the overlying Australian plate, GNS Science said the subducting Pacific Plate has started moving again.
Since the slow-slip event began in mid-September, experts have recorded six centimetres of movement at the plate boundary so far.
It is expected to continue for several more months.
Slow-slip events are common in New Zealand, most frequently occurring off the east coast.
The last time a slow-slip event occurred in the Kāpiti area was after the 2016 Kaikōura earthquake.
GNS Science said they can involve the same amount of movement on the plate boundary as a magnitude 7 quake, but they occur so slowly no one would know.
Small, local earthquakes can occur, but none have been detected at this stage associated with the slow-slip event.
GNS Science said the only way to measure and record slow-slip events is by looking at GPS data to see how the land is moving.
RNZ
