The earthquake struck 30km northwest of Levin at a depth of 37km at 7.53am this morning.
It was felt as far north as Auckland and near the bottom of the South Island, according to GeoNet.
Prime Minister Jacinda Ardern was live on television this morning, when the quake hit Wellington and she kept her cool throughout.
"We're just having a bit of an earthquake here," she said to Newshub host Ryan Bridge as the screen began to shake.
"Quite a decent shake here," she continued as the Covid-themed backdrops quivered behind her.
"The Beehive moves a little more than most," she quipped.
Newstalk ZB's Adam Cooper said it was a "very big quake".
"A rattler, went on for a long time," he said.
Geonet has revised the quake's depth to 37km, 30km northwest of Levin. Originally it said it was a 5.9 magnitude quake, and later revised that to 5.8.
It struck at 7.53am. It was categorised as "strong".
A number of aftershocks have occurred in the area as well.
All are being categorised as "weak" earthquakes on the GeoNet website. The latest reported a 3.7 magnitude quake 30km north-west of Levin at a depth of 31km.
All Metlink train services are being held at stations following the quake. Some will resume shortly but will run at reduced speeds, Metlink says.
Wellingtonian Mike Cagney had just pulled up to an intersection near the Wellington railway station when he thought "Why the hell is my car rocking?"He initially thought there was something wrong with his car or it was being buffeted by wind.
"It was jumping quite strongly and it was like holy cow - it was only later I realised it was an earthquake," he said.
The earthquake shook Parliament buildings and the Beehive for close to 10 seconds.
Most MPs don't return to Wellington until tomorrow morning. But the Prime Minister was in the Beehive - she had just finished a round of morning media interviews.
National MP Amy Adams was in a meeting at Parliament when it started to shake - causing all five people in the room to get under the table.
"We sort of all looked at each other and then everyone was getting under the table."
She did not expect it to last as long as it did - about 30 seconds, she guessed.
"It was a decent length and it certainly rolled around."
No one was hurt, she said, and nothing in the room had fallen.
Asked whether new National Party leader Todd Muller was in the meeting at the time, she said no.Adams said being from Christchurch, she had experienced and lived through "thousands" of earthquakes and dubbed that one "pretty strong."
It was longer than most - in the past, earthquakes are short and sometimes sharp. But this morning's lasted a lot longer than normal, the Herald's Jason Walls says.
It happened when many commuters were coming into work, navigating already wet Wellington roads.
Whanganui District Council Mayor Hamish McDouall said the 5.8 magnitude quake was pretty sudden and came in strong and hard.
"It was a very jittery quake and it got a bit stronger but I was very impressed the kids automatically went under the breakfast table."
He said it was the strongest earthquake he has felt since the Kaikoura earthquake in 2016.
McDouall said he had heard of no reports of damage so far in the Whanganui district.
"It's a good reminder that we live on fairly seismic active land and we just have to make sure we have enough water for three or four days and the things that can fall over are tied back."
Newstalk ZB's Scarlett Cvitanovich said it was felt in Whanganui and shook for around 30 seconds."This was the first significant earthquake I've felt since Christchurch and its many aftershocks. It arrived with a loud rumble in Whanganui and shook the house steadily for around 30 seconds," Cvitanovich said.
"It was long and rolling with a few sudden lurches but didn't lose its intensity."
Manawatū Guardian editor Merania Karauria, also in Whanganui, said it was "my first strong earthquake which seemed to go on forever. I am in an upstairs apartment and the shaking was scary. My cat ran to my bedroom. Being upstairs I was unsure where to stand. Items moved and I've moved bigger pieces onto the floor."
The Herald's Belinda Feek said it could be felt in New Plymouth quite strongly.
She said it was still shaking slightly after about 30 seconds.
A Wellington man told the Herald it was "quite a big earthquake ... I'm in Otaki and had to run to a door."
Fire and Emergency NZ has not received any quake-related calls. A police spokesperson said no reports of major damage had been received, but officers are making enquiries around the lower North Island to assess any damage to property.
In Kapiti, the quake was felt strongly. Rosalie Willis said "pictures fell off my walls" and said the shaking lasted about 10 seconds - with "a rumbling" continuing for a lot longer.
Kapiti News editor David Haxton, his wife and two kids quickly gathered together in the middle of their Raumati Beach house as the shaking gripped the house.
"The cat bolted, thankfully there was no damage or worse, but it certainly raised the heart rate of everyone," he said.
