Parliament sittings have been suspended for the rest of the week.
The news comes after Mr Key said the number of dead formally identified would rise during the day.
Police had told him 65 had been killed and so far about 38 had been identified.
"It's a very fluid number... as the day unfolds I am sure that formal identification number will rise from 38 -- very sad," he told TV One.
Labour leader Phil Goff told media that police had indicated to him a toll as high as 300 was possible.
Mr Key said the significant quake had a side-ways rather than vertical motion which had been devastating for buildings, even new structures.
"The number one focus has to be search and rescue."
New Zealand had accepted help from Britain, the United States, Singapore, Japan and Taiwan.
There were also concerted efforts to help displaced people and military aircraft would be taking groups to Wellington and Auckland.
Air New Zealand was offering $50 flights to and from Christchurch.
Mr Key said that the numbers trapped were hard to establish.
"Debris has fallen and potentially crushed a number of people... some reports are more positive than others about the responses they are getting from buildings."
Mr Key said specialist urban search and rescue people had to do the work because of the dangerous situation.
"I do want to pay...tribute to those people, and actually the thousands of Cantabrians that are working so hard to help us through this crisis.
"it's a remarkable effort from Cantabrians, and from people around New Zealand and indeed around the world."
He said people could help by donating cash and inviting Christchurch friends and relatives to stay as ongoing aftershocks were adding to stress and woes.
"But obviously we are extremely grateful for all the offers of help and we haven't been shy in accepting that help...we need to absolutely make sure we have as many people as necessary and we are working our way through that."
Trauma and grief counsellors were being despatched to help victims.
"This will be an issue of providing real support for people and giving them that confidence to regroup and rebuild over the months and years ahead and that's the extent of the process."
He said confidence of locals was badly damaged; "but we know that they are stoic people and they will get through this."
Another emergency Cabinet meeting would be held at 9am today.
