The country's entire east coast is at threat of dangerous waves, the New Zealand Civil Defense said on Twitter in an advisory, adding that initial waves may not be the largest and that they could continue for several hours.
"Move inland or to higher ground immediately," the warning said.
Nerve-racking footage of moments New Zealand earthquake struck
Strong magnitude 7.8 earthquake strikes near Christchurch, New Zealand
New Zealand is a South Pacific nation of islands, the two largest being the North and South islands, which are home to most of the population. Slightly more than 1 million people live in the South, while the North has more than 3 million.
'It's going to be a long night'
In Christchurch, memories of a devastating 6.3-magnitude earthquake in 2011 are still fresh for many. That quake killed 185 people and injured thousands, and reduced swathes of the city's historic area to rubble.
"The land has been very peaceful for many, many months. So this is bringing back all the rare memories," Chet Wah, owner of Designer Cottage B&B in Christchurch, told CNN.
"I just checked with all the guests. They are alright. It is scary. It's going to be a long night."