New Zealand's earthquake-devastated Canterbury region faces an almost one-in-four chance of another powerful tremor in the next 12 months, government scientists said Tuesday.

The region's major city Christchurch was rocked by a 7.0-magnitude quake in September and a 6.3 quake in February, the second of which killed 181 people and destroyed much of the downtown area.

GNS Science senior scientist Matthew Gerstenberger said aftershock modelling showed there was a 23 percent chance of a quake measuring 6.0-7.0 in the region over the next 12 months.

The assessment put the likelihood of a quake centred on Christchurch, New Zealand's second largest city, at about six percent, he said.

"This aftershock model that it's based on has been tested in a number of locations around the world and it's generally been quite consistent," Gerstenberger told Radio New Zealand.

Canterbury has been rattled by thousands of minor aftershocks since the initial quake in September and Gerstenberger said residents in the South Island region could expect them to continue.

"Certainly there will be more aftershocks ongoing for some time to come, so just be ready, be prepared for that," he said.

Both quakes occurred on faultlines that were previously unknown to scientists. GNS estimates the fault responsible for the September tremor had been inactive for 16,000 years before seismic forces caused it to move.

The February 22 quake, which caused damage estimated at NZ$15 billion ($12.3 billion), was the deadliest earthquake in New Zealand since a 7.8 magnitude quake in Hawke's Bay claimed 256 lives in 1931.

New Zealand sits on the so-called "Ring of Fire", the boundary of the Australian and Pacific tectonic plates, and experiences up to 15,000 tremors a year.