Christchurch, New Zealand: A powerful 7.8 magnitude earthquake rocked New Zealand early Monday, the US Geological Survey said, prompting a tsunami warning and knocking out power and phone services in many parts of the country.

The shallow tremor hit some 90 kilometres (57 miles) north of the South Island city of Christchurch which was devastated five years ago by a 6.3 tremor which killed 185 people in one of New Zealand's deadliest disasters.

Monday's quake, initially put at 7.4 but later upgraded, struck at 12:02am (1102 GMT Sunday). It was only 10 kilometres deep and felt throughout most of the country.

The main tremor was followed by a series of strong aftershocks and there were reports of damaged buildings in the small rural township of Cheviot near the epicentre.

‘It was massive and really long,’ Tamsin Edensor, a mother of two in Christchurch, told AFP, describing the powerful quake as the biggest since the deadly 2011 tremor.

‘We were asleep and woken to the house shaking, it kept going and going and felt like it was going to build up.’

In a brief message the Prime Minister John Key tweeted: ‘I hope everyone is safe after the earthquake tonight.’

The ambulance service said it did not receive any reports of quake-related injuries.

However, the national civil defence organisation, which is in charge of New Zealand's emergency management said a tsunami was possible.

‘The first wave activity may not be the most significant,’ it said in a bulletin, adding tsunami activity would continue for several hours.

Anna Kaiser, a seismologist with the GNS Science, said the government's earthquake monitoring service, the quakes were close to the coast.

‘They've been quite large. We've also seen a signal on the tide gauge at Kaikoura which is up to one metre (three feet) so it's reasonably significant, so people should take this seriously.’

However, the Pacific Tsunami Warning Center said that based on available data ‘a destructive Pacific-wide tsunami is not expected.’

In September, a strong 7.1-magnitude earthquake struck the east coast of New Zealand, generating a small tsunami, but no significant damage or injuries were reported.

New Zealand is on the boundary of the Australian and Pacific tectonic plates, which form part of the so-called ‘Ring of Fire’, and experiences up to 15,000 tremors a year.