A tsunami has hit after an earthquake struck New Zealand's South Island, reports BBC.

The US Geological Survey said the magnitude-7.8 quake hit just after midnight, some 95 km from Christchurch.

The tsunami arrived in the north-eastern coast about two hours later. Officials said the first waves may not be the largest, with tsunami activity possible for several hours.

Residents were warned to head inland or for higher ground along the coast.

A gauge at Kaikoura, 181 km north of Christchurch, measured a wave of two meters, 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 a wave would hit imminently by the civil defense.

Thousands of people have already evacuated their homes as aftershocks continue to hit the area, Radio New Zealand reported.

Others have taken to social media to offer people a safe bed for the night.

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.

The Herald newspaper said the tremor was felt all the way to Wellington, where sirens sounded and people fled buildings into the streets, some of them crying.

New Zealand's GeoNet suggests the earthquake may have been only a magnitude-5.5 tremor.

A resident of Christchurch said the tremor lasted a "long" time.

A magnitude-7.1 quake struck 169 km north-east of Gisborne on the North Island in September, sparking a tsunami warning. It caused some damage to property, but no injuries.