A severe earthquake has shaken the Wellington region this afternoon.
After its initial report, GNS reported on its website that the quake was 6.2, and only 8km deep, 10km southeast of Seddon, near the coast in Marlborough.
GNS seismologist Bill Fry said it was not immediately clear what the exact magnitude of the shake was as data continued to come in, but it was likely to be "in the upper 6s''.
He said it was a "very strong, shallow shake'' which was widely felt.
The strongest shaking was recorded in Wellington.
It was probably a bit to the south of the quakes of two weeks ago.
"Anecdotally, I was sitting in my office chair in the same spot and I thought this had longer duration than the shakes of two weeks ago. "Scientifically, if it gets above the magnitude of the largest one two weeks ago then this becomes the main shocks and those become foreshocks.''
By that, he meant that this could be "the main event'' and the previous shakes could have been leading up to this.
The large quake has been followed by a series of aftershocks.
A further significant aftershock struck about 3.10pm, about 45 minutes after the initial shake and fire engines were still racing through the streets of the CBD to call outs.
A house had almost totally collapsed just south of Lake Grassmere in Marlborough. There were no injuries.
It was earlier reported the house was in Nelson.
There was also reports of a a rockfall on State Highway 1, just north of Ward.
Police said large boulders had come down on parts of SH1.
KiwiRail said upper South Island freight movement was also likely to have been put on hold.
Phone connections into the capital are disrupted, but a spokeswoman was hoping for text updates within the next hour on any damage to rail infrastructure such as tunnels, bridges, overhead power lines and - in the case of the inter-island ferries - wharves at Picton and Wellington.
NZ Bus later said Wellington buses were running again.
ANZ said its Wellington contact centres were evacuated.
Wellington had also lost cellphone coverage.
Ambulance central communications said they had yet to receive any calls related to the earthquake.
TranzMetro said it had immediately stopped all train services in the capital. Wellington's railway station had been evacuated.
Wellington Airport's runway has been inspected and is back open, although travelers are advised to check with their airline or airport website for flight status.
No tsunami threat has been issued.