Key Points:

Thousands of people were left shaken following a massive earthquake tonight which was felt throughout the lower North Island and Upper South Island.

The quake, which measured 6.8 on the richter scale, was centred under the sea 50km south-east from Gisborne but was felt all the way down to Christchurch.

While the extent of the damage was not known last night, firefighters were flooded with calls about fallen power lines, building damage and activated alarms.

Newstalk ZB's Gisborne reporter Murray Robertson says police have announced they have no reports of any injuries so far, but the town is in a state of chaos. He says the main street of Gisborne is shut, as the roofs of several large buildings have collapsed, and one of the main roads into the city has a huge hole in it.

Mr Robertson says off duty police have been called into work and Civil Defence staff are at the scene. He says police are asking everyone not to drive and to return to their homes.

Power in Gisborne is now back on, after Gisborne was plunged in to darkness for almost 40 minutes after the quake.

Gisborne man Merv Bradley, 78, said he had been living in the area since 1944 and it was the worst he had felt yet.

"It struck at about 9pm, I just had to ride it out. There are books and ornaments all over the place and there will be a massive cleanup in the morning."

Auckland man Norman Mercer was on the phone to a friend in Gisborne when the woman, who is in her 70s, started screaming "help me".

Mr Mercer said he heard crashing in the background before the line went dead. He tried to ring her back but was unable to make contact, leaving him concerned for his friend's wellbeing.

A Whakatane man, who only wanted to be known as Aaron, was watching television when the quake struck.

"It was a good rolling motion. It went for more than ten seconds. I thought 'oh no this could go for a while and I was getting my butt under a doorway"'.

GeoNet duty seismologist Warwick Smith said thousands of people had logged on to the GeoNet website from all over the country to record the fact they had felt the quake.

Mr Smith said there was minor damaged reported in Gisborne, such as broken glass and ornaments, but there were no initial reports of any injury.

He said an earthquake of a 6.8 magnitude was rare.

"It was about the same size as the 1987 Edgecombe earthquake but this one was offshore so there was nobody sitting right on top of it, although it could be felt quite strongly."

Mr Smith said GeoNet would be studying the earthquake more over the next few days.

Gerry Browne of Opotoki said that during the quake the water was being sloshed out of his swimming pool.

"We have got a lot of damage and are still assessing the situation," a police spokesman from Gisborne said shortly after 9pm.

A central police communications spokesman said "the quake has caused quite a lot of damage in the Gisborne area".

He said the city's central business district lost its power and there was minor damage to some buildings with "lots of glass on the streets".

"There were holes opening up in some streets, partial building collapses in some areas."

Civil defence had been activated and they were carrying out emergency plans.

"There have not been a lot of injuries reported yet but a contingency plan was being activated at the hospital."

Tairawhiti District Health Board CEO Jim Green issued a statement urging seriously ill or injured patients to seek treatment at Gisborne Hospital.

"The hospital is operating under its emergency plan with senior staff at the hospital or on standby to manage any patient demand - so far there have been no admissions with injuries resulting from the earthquake," he said.

Mr Green said that people with minor injuries or ailments should see their GPs.