Thousands of people along New Zealand's east coast fled their homes to head for higher ground as emergency services warned of destructive waves of up to five metres (yards) in some areas.
"The first waves have arrived but we know that it is too early to say what the impact has been," said Sarah Stuart-Black, national controller at the Ministry of Civil Defence. "Our concern is what is coming. Future waves are coming may be bigger than what has come before."
St. John Ambulance said it was sending helicopters carrying medical and rescue personnel to near the epicentre of the quake, 91 km (57 miles) north-northeast of Christchurch, where a 6.3 quake in 2011 killed 185 people, on New Zealand's South Island.
"We are starting to get reports of injuries from the earthquake area, including Culverden area and Kaikoura," Dion Rosario, St John Incident Controller, said in a statement. "However, at this stage we do not have specifics."
Police said they were investigating reports of a collapsed building in the coastal tourist town of Kaikoura.
The first tremor and a series of aftershocks were felt throughout most of New Zealand. There were no immediate reports of deaths.
"The whole house rolled like a serpent and some things smashed, the power went out," a woman, who gave her name as Elizabeth, told Radio New Zealand from her home in Takaka, near the top of the South Island.
In Wellington, there was gridlock on the roads to Mount Victoria, a hill with a lookout overlooking the low-lying coastal city.
"I'm just sort of parked by the side of the road and I think people are trying to go to sleep the same as I am," Wellington resident Howard Warner told Reuters after evacuating his seaside house.
Richard Maclean, a spokesman for the Wellington City Council, said there was structural damage to several buildings.
"We've got reports of broken water pipes and lots and lots of things off desks and shelves," he told Radio New Zealand.
Other residents reported glass had fallen from buildings into the streets and hotels and apartment buildings had been evacuated.
A spokeswoman for the city council of Christchurch, where tsunami sirens rent the air, said its "emergency operations centre" was active.
Several roads around the South Island were closed and damage to power and phone networks was reported.
Pictures shared on social media showed buckled roads, smashed glass and goods toppled from shelves in shops in Wellington and the upper South Island.
New Zealand's Geonet revised up its estimated magnitude of the quake to 7.5, from 6.6 earlier.