The epicentre was 55km (35 miles) north-west of Christchurch, at a depth of 12 km (7.5 miles), it added.

Police said there was widespread damage to buildings and roads as well as power cuts. Two men were seriously injured by falling masonry and glass.

A state of emergency was later declared in Christchurch, New Zealand's second largest city with a population of 386,000.

New Zealand lies at the southern end of the so-called Pacific Ring of Fire, and above an area of the Earth's crust where the Pacific Plate converges with the Indo-Australian Plate.

The country experiences more than 14,000 earthquakes a year, of which only around 20 have a magnitude in excess of 5.0.

The last fatal earthquake was in 1968, when a 7.1-magnitude tremor killed three people on the South Island's western coast.