Business owners in central Christchurch are anxious about their shops being sealed away from customers behind cordons a week after the earthquake.

The mayor of Christchurch says developers built on land that could turn into 'murky soup' after an earthquake, despite opposition from the council.

Aftershocks rattling Canterbury are likely to last for "weeks" rather than days but will get less frequent and probably weaker.

The Christchurch Mayor has warned the rest of NZ that they are not immune from the type of natural disaster that struck his city this week.

A second big earthquake in three days sent lower North Island residents scrambling for cover.

When the tremors return, residents at one Christchurch rest home band together with cups of tea and company.

At least 22 commercial properties and 50 homes in the small quake-damaged North Canterbury town of Kaiapoi have been deemed unsafe, with its mayor admitting the town is changed forever.

Most schools in the quake-devastated Canterbury region are not likely to open until Monday, the Education Minister says.

The final bill for the Christchurch earthquake may reach $4 billion, almost double initial estimates, Treasury Secretary John Whitehead said afternoon.

Aftershocks are continuing to hit Canterbury with one measuring 4.1 the latest in a series today that also included a 5.1.

Mayor Bob Parker says Christchurch city is coping "amazingly well" with the continuing fall-out from Saturday morning's earthquake.

The Government has set aside $2.4 million for trauma counselling for Christchurch earthquake victims, Paula Bennett says.

The state of emergency in Christchurch has been extended a further seven days following an aftershock of 5.1 on the Richter scale this morning.

Stricken homeowners and workers will start receiving compensation and emergency payouts within days as Christchurch's earthquake bill threatens to exceed $2bn.

Rebuilding up to 100,000 damaged and destroyed homes in quake-hit Canterbury could take years, a building executive says.

Mayor Bob Parker is unsure about the comparison with former New York mayor Rudy Giuliani, but the earthquake that struck Christchurch could help him keep his job.

Christchurch was "inevitably going to be brought to its knees" by an earthquake because of the nature of the ground it stands on, a scientist says.

"It's essentially a ghost town. You can see utter devastation," said Prime Minister John Key.

"It was like a giant had picked up our house and was just shaking it, shaking and shaking," said one resident.