More than 20 aftershocks have been felt since a "severe" magnitude 5.7 earthquake rocked Wellington and the upper South Island this morning, and GNS says more are to come.
Emergency service have not received any reports of damage but workers in central Wellington have described multi-storey office buildings swaying for at least 30 seconds when the quake hit at 9.06am.
GeoNet said it struck 30km east of Seddon, in Marlborough, at a depth of 8km.
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 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.
KiwiRail placed speed restrictions on trains going through tunnels or over bridges following the quake.
A spokeswoman said all structures and lines had since been checked and no damage found so the restrictions had been lifted.
The quake shook the emergency services communications centre on the seventh floor of the police station on Victoria St in central Wellington for a good 30 seconds, a Fire Service central communications spokeswoman said.
Upper Hutt woman Deb Norman said the first shake set the neighbour's dog barking, before a second shake hit less than a minute later.
"[It was] increasing in intensity and caused our three dogs to go crazy! The shake lasted in our home based in Upper Hutt for two and a half minutes.
"Oh well, was time for our teenage to get up - shake, rattle and roll; up time!"
Katie Russell, who was in Johnsonville, said the chair she was sitting on moved side to side.
"So my heart's still racing! Scary ... I've felt a couple of mild aftershocks too."
Twitter was flooded with tweets about the quake in the minutes afterwards.
GeoNet tweeted: "Well we all felt that at the office!"
Staff at the Ministry of Justice call centre in Wellington were "screaming in the background" when Press reporter Blair Ensor was on the phone to them when the quake struck, he tweeted.
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 8 years."
Ashleigh Lambert tweeted the quake was "scary".
"Phew, that was the first time I've actually considered getting under my desk for an earthquake."