The US Geological Survey said the quake hit just after midnight, around 95 km from Christchurch, BBC reported.
Officials who have warned the residents to head inland or for higher ground along the coast, said the first waves may not be the largest, with tsunami activity possible for several hours.
A gauge at Kaikoura, 181 km north of Christchurch, measured a wave of two metres, according to Weatherwatch.co.nz.
Smaller waves are said to be arriving in Wellington and other areas, the website said.
Residents in the Chatham Islands, an archipelago 680 km south-east of the mainland, were also being warned by the civil defence authorities that a wave would hit imminently.
Thousands of people have already evacuated their homes, Radio New Zealand reported.
New Zealand lies on the notorious Ring of Fire, the line of frequent quakes and volcanic eruptions that circles virtually the entire Pacific rim.
Christchurch is still recovering from the 2011 earthquake that killed 185 people and destroyed the city centre.
According to the Herald newspaper, the tremor was felt all the way to Wellington, where sirens sounded and people fled buildings into the streets, some of them crying.
Early reports suggest some houses in Cheviot town, near the epicentre, have been damaged.
But contrary to reports by the US Geological Survey, New Zealand’s GeoNet suggests the earthquake may have been a 7.5 magnitude tremor.
A resident of Christchurch said the tremor lasted a “long” time.
On Twitter, Hayley Colgan described it as “the most terrifying earthquake I think I’ve felt in my 23 years in New Zealand”.