A powerful earthquake slammed New Zealand's already-bruised city of Christchurch on Tuesday, killing at least 65 people, and sending rescuers scrambling to help people trapped under collapsed buildings.

The 6.3 magnitude earthquake hit the country's second-largest city around lunchtime, collapsing buildings and sending bricks and other heavy debris toppling into busy city streets. It was the second major quake to strike Christchurch in the past five months.

Prime Minister John Key told reporters that the death toll was at least 65, and was expected to rise further.

Video footage showed some multi-storey buildings collapsed in on themselves, and others with walls that had collapsed into the streets, strewn with bricks and shattered concrete.

Sidewalks and roads were cracked and split, and thousands of dazed, screaming and crying residents wandered through the streets as sirens blared.

Groups of people helped victims clutching bleedings wounds, and others were carried to private vehicles in makeshift stretchers fashioned from rugs or bits of debris.

Christchurch Mayor Bob Parker declared a state of emergency and ordered people to evacuate the city center. Troops were deployed to help people get out and to throw up a security cordon around the stricken area, Deputy Prime Minister Bill English said.