Part of the earthquake-stricken New Zealand city of Gisborne remained behind barricades and under a state of emergency on Friday night as civil defence chiefs told residents to check on their neighbours nearly 24 hours after a disastrous shake.

An elderly woman, who reportedly suffered a heart attack just after the quake struck on Thursday evening, was the only known fatality of the tremor which caused havoc in the city centre, collapsing two historic buildings and severely damaging another seven blocks of shops and offices.

It was the most damaging shake in New Zealand - one of the world's most earthquake-prone countries - for 20 years and although centred off Gisborne, a port city on the east coast of the North Island, it was felt 1,250 kilometres away in Dunedin, in the South Island.

About 20 people were treated for minor injuries from flying glass and debris, and officials said it was a miracle that nobody was killed as bits of masonry fell from buildings into central city streets.

Civil Defence chief John Hamilton said, "There have been no serious injuries reported from last night's earthquake but there might still be people who have not been able to contact anyone."

As police allowed some retailers back into their stores which inspecting engineers judged safe from the threat of falling glass or concrete, others were told their premises were too dangerous to allow them to resume trading.

After seismologists consulted scientists in the United States Geological Survey, the strength of Thursday's night quake was downgraded from 6.8 on the Richter scale to 6.6.

It was still big enough to send hundreds of Gisborne residents, aware of the danger of a tsunami, to their cars to find higher ground.

Scientists said they had done the right thing although the shake was too deep, at 40 kilometres below the surface, to create a tsunami.

Merv Bradley, a pensioner resident, told Television New Zealand the experience was "the feeling you were being shaken by some giant - the force was brutal."