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.

An open source online map has been set up to help Cantabrians find help nearest to them.

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.

The Christchurch earthquake is expected to provide a much-needed lifeline for NZ's beleaguered construction sector.

"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.

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