It was the second quake to hit the city in five months, and New Zealand's most deadly natural disaster for 80 years.

"We may well be witnessing New Zealand's darkest day...The death toll I have at the moment is 65 and that may rise," New Zealand Prime Minister John Key told local TV.

"It's hard to describe. What was a vibrant city a few hours ago has been brought to its knees," added Key, who had flown to Christchurch.

The 6.3 magnitude quake struck at lunchtime, when the streets and shops thronged with people and offices were still occupied.

Christchurch's mayor described the city of almost 400,000 people as a war zone.

"There will be deaths, there will be a lot of injuries, there will be a lot of heart break in this city," Mayor Bob Parker told Australian TV by phone. He later told local radio up to 200 could be trapped in buildings.

The quake is country's worst natural disaster since a 1931 quake in the North Island city of Napier which killed 256.