There has been a flurry of smaller shocks since the initial magnitude 5.7 quake, the strongest recorded as magnitude 3.8 at 9.38am.
At least 10 other shocks were recorded in the Marlborough area by 9.45am.
A Fire Service central communications spokeswoman said there were no reports of damage in the Wellington region so far, although an alarm activation may have been caused by the quake.
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, she said.
There have also been no reports of damage in the upper South Island, a Fire Service southern communications spokesman said.
GNS duty seismologist Anna Kaiser said the quake, centred halfway between Wellington and Blenheim, was the biggest in the region this year.
It was felt as far north as New Plymouth and as far south as Canterbury, with more than 6000 people reporting they felt the quake on the GeoNet website.
Dr Kaiser said that level of feedback was "quite a lot'', due to the quake striking close to a large urban centre.
There have been more than 20 aftershocks in the five hours since the initial quake - and more are expected.
Most of the aftershocks would not be felt, but some were able to be felt in Wellington and at the top of the South Island.
"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.''
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.
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."
FACEBOOK QUAKE COMMENTS:
Miles Lacey: "I'm in Paraparaumu Beach. It was a stronger and longer shake than normal but not enough to get me out of bed or to disturb anything on the shelves. Then again my tallest furniture is rickety and tends to move with the shaking earth."
Eleanor Morgan: "Felt it in Renwick just out of Blenheim. It came in two waves, and was very loud. After going through all the Christchurch Earthquakes it was a nasty reminder!! Really hope that is the biggest we get up here, I don't think I could go through it all again!"
Shenah Rowe: "Felt here in Paraparaumu.... Hit our house hard...we have slight damage all over the house, from ceiling tiles lifting, to one wall has lifted up about 4cm off the floor...haven't checked out side yet as dealing with a sick 3 yr old at the moment."
Stefanie Halliwell: "Pretty big one felt here in Wainuiomata. Wondered when it would stop. Whole house shaking. Seemed to last a wee while."
Nicola-Jane Young: "I did. West Coast of south island, east of greymouth."
Angela Pennington: "Felt it in Wellington city! Big fright after chch"
Read more Facebook comments here.
HOW TWITTER REACTED TO THE QUAKE:
That was a decent jolt/roll. Hopefully no serious damage. A good reminder to have plan and keep the #eqnz kits up to date. #sermonforday
Base isolaters in our building make for that weird swaying feeling, a shaky wake up to my Friday #eqnz
Being on the 30th floor NOT cool when that blimin earthquake hit building moved #eqnz - just as well #eqnz has meeting on same floor as us
So. Thats the first time I have felt the need to drop, cover & hold. Quite the jolt then rumbling then the building swayed. #eqnz :-(
Well we all felt that at the office!
On the phone to Ministry of Justice staff in Wellington when #eqnz struck. Staff screaming in the background.
Oh, that was a shaker! Good to know two adults and two small children all fit in one doorway #eqnz #wellington
Sitting in a hotel room on the terrace in Wellington on the 5th floor the room certainly got a sway on
I'm all shook up. My first decent Wellington quake. No longer a quake virgin. 5.7, says Geonet. http://t.co/ikSlwvPYW7
That's the biggest quake I've felt in Wellington since Gisborne in 2007 #eqnz
MAP:
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