GNS Science seismologist Anna Kaiser said there had been more than 20 aftershocks since the main earthquake.
The largest, a magnitude 4.5, occurred at 3.21pm centred 35km east of Seddon at a depth of 16km. It was also widely felt.
"There is expected to be increased seismicity in that region in the next few days," Dr Kaiser said.
"They will taper off over the next few days - there will be much fewer than there have been today.
"We always get a level of background activity in this area anyway, so we do expect small quakes for a while."
More than 6000 people had reported feeling the main earthquake, from as far north as New Plymouth to Canterbury in the south.
Dr Kaiser said a number of known faults in the area had been mapped by the National Institute of Water and Atmospheric Research.
However, it was not yet known which fault the quake had occurred on.
The quake shook the emergency services communications centre on the seventh floor of the police station in Victoria St in central Wellington for a good 30 seconds, a Fire Service central communications spokeswoman said.
Twitter was flooded with tweets about the quake in the minutes afterwards.
GeoNet tweeted: "Well, we all felt that at the office!"
TVNZ reporter Simon Bradwell tweeted that the quake was felt strongly in its fifth floor newsroom.
"Strong enough for most to get under their desks, first time I've seen that in over eight years."
