Several foreign nationals are confirmed to have died in the earthquake and police are still trying to establish the number missing.
Two British and two Irish people are confirmed to have died.
The Chinese Embassy says it has grave concerns for 20 students trapped in the Canterbury Television building, which housed an English language school.
Ten Japanese students who were inside the building are also unaccounted for.
The British Foreign Office has confirmed that two British people were killed. A spokesperson says their families have been told and the British government is providing consular support.
Ireland's Department of Foreign Affairs has confirmed two Irish men were killed in the earthquake.
One man, in his 40s, was believed to have been in the Pyne Gould building. He was married with children and had been living in New Zealand for some time.
The department confirmed on Wednesday evening that another man, Eoin McKenna, 41, was killed when his car was crushed by falling debris. He was married to a New Zealand woman and also had two young children.
Superintendent Russell Gibson says a police team is still working through information to establish how many foreign nationals are missing.
Foreigners asked to phone home
Police are asking people from other countries who are visiting New Zealand to contact their families.
Superintendent Dave Cliff says all visitors - not only those who were in Christchurch during the quake - should call home.
Mr Cliff says the Red Cross will then be able to remove names that should not be on the list of people suspected missing or dead.