At least 75 people have died in Tuesday's earthquake in New Zealand, causing Prime Minister John Key to declare a national state of emergency.

According to the police, there is "incredible carnage right throughout the city", with "bodied littering the streets", the BBC informed on Wednesday.

More than 300 people are still missing, while 48 were pulled out from collapsed buildings alive during the rescue operations overnight.

The authorities have imposed a night-time curfew in the areas that were worst-affected by the 6.3-magnitude earthquake, which struck New Zealand's city of Christchurch at a shallow depth of 5 km at 12:51 pm local time.

PM John Key, who grew up in Christchurch, declared a national state of emergency after a cabinet meeting on Wednesday. In his words, it would allow the greatest possible coordination of local, national, and international resources to work on rescue and recovery efforts.

"Things will get better. Christchurch will rise again. This country is right behind you and we are backing you with all of our might," Key said.

The rescue operations included more than 500 search and rescue personnel, police, fire service staff, soldiers and volunteers. They have been trying to find survivors under the rubble and many of them have been using their bare hands.

"There are bodies littering the streets, they are trapped in cars and crushed under rubble. We are getting texts and tapping sounds from some of these buildings and that's where our focus is," police officer Russel Gibson said.

New Zealand experiences more than 14,000 earthquakes a year, of which only around 20 have a magnitude greater than 5.0.

Tuesday's was the county's worst natural disaster since a 1931 quake in the Hawke's Bay on the North Island, which killed 256 people.