The new tremor struck about 13:45pm local time (00:45 GMT) at a depth of 10km (6 miles), northeast of Christchurch.
A 7.8-magnitude quake, with the same depth, hit the same area just after midnight, triggering tsunami warnings.
Authorities are rescuing and evacuating residents along the east coast.
There have been hundreds of strong aftershocks, causing power shortages and cuts in water supply.
Waves of around 2m (6.6ft) hit the coast shortly after the first earthquake. Authorities have since lifted tsunami warnings for some areas.
(Pics : Dailymail)