Violent aftershocks rattled Christchurch Tuesday as parts of the city re-opened for the first time since New Zealand's biggest quake in nearly 80 years.

The official geological monitoring service GeoNet reported more than a dozen aftershocks overnight, two measuring magnitude 5.4, prompting authorities to evacuate 70 people sheltering in temporary accommodation in a high school.

Christchurch mayor Bob Parker said the aftershocks were further damaging weakened buildings in the South Island city of 340,000 people and playing on residents' frayed nerves.

Parker told national radio he checked his bags were packed when the latest seismic jolts hit because he had an "ominous feeling" they were going to get worse.

The civil defence ministry warned that aftershocks measuring up to magnitude 6.0 would remain a danger over the next week.

GeoNet also reported a separate 5.2 magnitude earthquake near Hawke's Bay on the North Island but said there was no damage or injuries. The area was the scene of New Zealand's most destructive quake in 1931, which killed 256 people.

In a sign that life was slowly returning to normal in Christchurch, a no-go zone in the central business district was significantly reduced, allowing residents back into areas previously off-limits.

Police and the army had sealed off the entire city centre after Saturday's 7.0 magnitude quake toppled building facades and littered the streets with debris and broken glass.

Police warned that people still needed to be aware of the danger from falling masonry and other debris in the newly opened area.

"We ask that people continue to be vigilant and put safety first when entering buildings or the cordoned area," inspector John Price said.

No one was killed in the weekend quake but many residents have recounted close shaves.

Christchurch will remain in a state of emergency until midday Wednesday (0000 GMT) and the education department said schools would remain closed for the rest of the week.

The civil defence ministry said electricity had been restored to most of the city and all major roads and rail links were open.

But about 300 people whose homes were uninhabitable were sheltering in welfare centres and residents were still being advised to boil water due to the risk of contamination from burst sewage pipes.

"Public health remains the main area of concern, with damage to sewerage and water systems expected to cause issues for at least another week," the ministry said in a statement.

Prime Minister John Key cancelled a trip to Europe, which was to have included a visit to Queen Elizabeth II's Scottish residence Balmoral, as the clean-up in Christchurch continues.

"There's a job to be done and I'm going to go down there and be part of it," said Key, who announced compensation for workers left out-of-pocket because they were being forced to stay at home.

With an estimated 100,000 homes damaged, Christchurch City Council said some buildings would have to be demolished, including a seven-storey office block built in 1906 which was the city's first high-rise.

On the city's eastern outskirts, authorities began evacuating inmates from one of the country's biggest jails, Christchurch Men's Prison, after the quake disrupted water supplies to emergency sprinkler systems.

"We are taking the precaution of moving prisoners to avoid the risk of any further pressure on local emergency services in Canterbury," a Corrections Department spokeswoman said.

In a boost to rugby supporters, the operators of AMI Stadium said the venue escaped structural damage in the quake and would host Rugby World Cup matches next year as planned.