Civil Defence Minister John Carter says the Christchurch earthquake is likely to be the worst disaster New Zealand has ever faced and the only time the country has declared national state of emergency.
At least 75 people are known to have died in Tuesday's 6.3-magnitude quake.
The declaration provides the Government with sweeping powers to ensure people's safety and to provide all the support needed.
Mr Carter says past disasters have been declared local or regional emergencies.
Although the number of fatalities and the amount of damage is still unclear, by today's standards, it is the worst disaster of recent times, he says.
The Government says it will constantly review whether it needs to keep the state of emergency in place.