A formal curfew is now officially in place in the Christchurch central business district until 7am and those who breach it may be arrested, say police.

Geologists say today's earthquake had an epicentre 10km south-east of Darfield, but so far they have not been able to link it to a known fault line.

Prime Minister John Key has completed a flyover of Christchurch in order to assess the damage wreaked by this morning's earthquake.

Photos from the 7.1 magnitude earthquake in Christchurch this morning.

Craters opened up at a Kaiapoi motorcamp during this morning's earthquake, with caravans sinking up to their axles.

The cost of damage from today's devastating quake could be as much as $2 billion, Earthquake Commission chief executive Ian Simpson says.

Services and infrastructure in Christchurch have been seriously affected by this morning's quake. Here is the current situation.

Read the facts about this morning's quake, Christchurch's faultlines and the many aftershocks that have rocked the city.

An historic pub in the South Canterbury town of Temuka has been badly damaged by today's earthquake and the proprietor is unsure when it will reopen.

It struck in the wee small hours, a rumbling roar and intense swaying that lasted for the best part of a minute.

Christchurch Mayor Bob Parker says there is "significant structural damage" to his city after this morning's 7.1 quake, with some eastern suburbs facing possible evacuation.

Vodafone is advising customers in Christchurch to keep cellphone calls sort, or preferably use SMS messages.

The magnitude 7.1 earthquake which struck Christchurch about 4.30am today rates among the top dozen to hit New Zealand.

Christchurch residents are recounting terrifying ordeals during this morning's 7.1 magnitude earthquake.

The Govt will spend $3.5m to upgrade tsunami warning systems across the Pacific, Murray McCully announced today.