Prisoners are being evacuated from one of New Zealand's biggest jails after a major quake disrupted water supplies to emergency sprinkler systems, says an official.

A Corrections Department spokeswoman said inmates would be bused or flown from Christchurch after Saturday's 7.0 earthquake affected water supplies, sending pressure in the fire sprinkler systems plummeting.

"There is no structural damage to the prisons, just a concern that if there was a fire, that there wouldn't be sufficient water pressure to have the sprinkler system performing effectively," she told AFP.

"We are taking the precaution of moving prisoners to avoid the risk of any further pressure on local emergency services in Canterbury."

A state of emergency was declared in New Zealand's second-largest city after Saturday's quake felled buildings, bent railway lines and tore deep rifts in roads and fields.

The spokeswoman said there was adequate drinking water, sanitation and staffing at both Christchurch's men's prison and the smaller women's jail.

She said some inmates would be transferred in coming days as a "temporary measure" while contractors carried out repairs, without revealing how many prisoners were involved or their security classification.

Men would be taken by bus to a jail in the nearby town of Otago, while female prisoners would be flown to a women's facility in the North island city of Auckland on charter jets.

"All prisoner movements will follow robust escort processes to ensure the safety of the public is not compromised," she said.

"Once we have assurance that the water pressure has stabilised, we will return prisoners to the region."

Christchurch men's prison is one of the country's largest, housing almost 1, 000 inmates, while the women's prison is home to 138 prisoners. Both hold offenders classed from minimum to high-medium security.

At nearby Rolleston Prison, south of Christchurch, it was "business as usual and largely unaffected by the earthquake," the spokeswoman added.