The city of Christchurch, New Zealand, has been hit by a further set of powerful aftershocks following a 7.1 earthquake at the weekend that caused widespread damage.

More than a dozen aftershocks struck overnight – two measuring a magnitude of 5.4 – which further weakened buildings damaged by Saturday’s earthquake.

Two-thirds of the city’s 160,000 homes were reported to have been damaged, although there have been no fatalities.

A state of emergency has been extended until Wednesday, and the city centre remains cordoned off. Officials have warned tourists against all but essential travel to the city.

Experts warned that more tremors were likely.

“It is still possible that we will have a magnitude six in the next week,” said Ken Gledhill, a monitor at the geological agency GNS Science.

“People ought to be aware of that, particularly if they are around structures which are already damaged. For a shallow earthquake like this, they will go on for weeks,” he added.

Some of the city’s most historic buildings are among those having to be pulled down because they are beyond repair.

New Zealand’s AMI Stadium sustained no structural damage in the earthquake and the venue operators said it would host Rugby World Cup matches next year as planned.

About 300 people left homeless by the quake have been sheltering in welfare centres.

Prime Minister John Key has warned that New Zealand’s economic recovery will suffer because of the earthquake.

New Zealand lies at the southern end of the so-called Pacific Ring of Fire, and above an area of the Earth’s crust where the Pacific Plate converges with the Indo-Australian Plate.

The country experiences more than 14,000 earthquakes a year, of which only about 20 have a magnitude in excess of 5.0.