Civil Defence Minister John Carter has said that today's declaration of a National Emergency is the first time the measure has been invoked, and that reflects the likelihood yesterday's quake may prove to be New Zealand's worst natural disaster.

While the current Civil Defence Act only came into effect nine years ago, previous legislation allowed for a similar declaration, but that had never happened, said Mr Carter.

In fact, a state of emergency has been declared before - by Prime Minister Holland during the 1951 waterfront dispute.

But this is the first time a state of emergency has been declared for a civil defence emergency.

"We felt it was justified in this case because of the devastation that's occuring and the likely impact it will have on so many people."

Asked whether this meant the quake was New Zealand's worst natural disaster, Mr Carter said: "Certainly history would tend to show that it's likely to be".

"Until we have an absolute understanding on what's likely to be out there we can't say that for sure."

The 1931 Napier earthquake killed 256 people but Mr Carter said until the death toll and damage from yesterday's quake was confirmed, "it's difficult to make those sorts of comparisons."

Mr Carter said the confirmed death toll so far, including those dead who remained unidentified, was still at 75. He wasn't prepared to give any indication of how high it may climb.

"I don't think it would be appropriate for me or any one else to speculate on that."

Mr Carter was not prepared to give a number of those who are missing either.

"There are a number of people we know were in hotels and who may just have gone."