The New Zealand South Island city of Christchurch Mayor Bob Parker said on Wednesday that the official death toll was 75 after Tuesday's massive earthquake.

Rescuers work to help the injured people after the earthquake. An earthquake measuring 6.3 on the Richter Scale jolted South Island of New Zealand at 7:51 a.m. Beijing Time Tuesday, causing widespread destruction in the city center. [CFP]

There are also 300 people believed to be missing but details are unclear and officials are currently trying to refine that list.

The Prime Minister declared a national state of emergency after an emergency cabinet meeting on Wednesday.

He said the state of emergency "enables the strongest possible focus of local, national and international resources working together to achieve the best possible response in the shortest time".

Civil Defence director John Hamilton will be able to exercise control over the functions, duties, and powers of emergency management groups throughout the country.

Key said there were no other differences between the powers under the existing state of local emergency and a national state of emergency.

Those powers would be exercised working in close support of and cooperation with Bob Parker, and the Christchurch Civil Defence team.

Hundreds of search and rescue personnel, fire service staff, army personnel and volunteers worked under floodlights through the night and morning to get to those who are still trapped -- some of whom have been texting for help.

Search and rescue teams are focusing on 10 critical buildings where people may be trapped.

The fire service's head of special operations, Jim Stewart- Black, said more than 200 search and rescue personnel were on the ground, and that number would swell to 700 in the next 48 hours as reinforcements from all over the world.

Bob Parker said the central city would be shut down on Wednesday and would stay that way for a significant amount of time.