At least 75 people are now known to have been killed during the powerful 6.3-magnitude quake, which struck around six miles outside the city of Christchurch on the South Island during lunchtime rush hour.

"We'd just ordered coffee in a shopping mall and heard a rumble similar to an articulated truck going past. The cutlery and crockery started to rattle and then the building and the earth started to go from side to side," Gary Millar, from Ballymena, told UTV.

"People screamed and panicked, but we sat still. There was then a fairly major aftershock and I had to grab a bench at the side of the road to stop myself from falling over, it was so strong."

He was left shaken by his experience close to the epicentre, where widespread destruction and injury was caused.

This may be New Zealand's darkest day.

John Key, New Zealand PM

It is understood 300 people are missing, while as many as 100 people remain trapped under the rubble of buildings in the stricken city, where rescue teams are working round the clock in a bid to free the survivors.

Serious concerns have been raised over the whereabouts of five Irish people. The Department of Foreign Affairs confirmed about 200 Irish citizens are believed to have been in the Christchurch area when the tremor struck.

A spokesman said while there was general concern for about 70 of those, it had serious concerns for five.

A search and rescue team has been deployed from the UK to help with the rescue efforts and Prime Minister David Cameron has said the UK is poised to assist however possible.

"The people of New Zealand have been hit by a devastating earthquake, not once, but twice in a matter of months, and I want to pay tribute to their resilience. They have our deepest sympathies and condolences," he said.

The spire of the city's stone Cathedral toppled into a central square during the quake, while buildings collapsed and roads cracked.

Gareth McGoldrick from Omagh also described what happened, saying: "The place where we were staying - the building just started to shake at 12.51pm. We stayed by the door-frame and then the building started to sway."

Families from Northern Ireland are trying to make contact with loved ones living or holidaying in New Zealand.

Margaret Leigh, New Zealand Consul for Northern Ireland, said: "If anyone here has family of relatives they're concerned about, the MOD website is running up to the minute information.

"Civil defence people have asked if you could text or email as the phone lines are overloaded and this minimises their ability to get to emergency phone calls."

In a message of support sent to New Zealand Prime Minister John Key, the Queen said: "I have been utterly shocked by the news of another earthquake in Christchurch.

"Please convey my deep sympathy to the families and friends of those who have been killed; my thoughts are with all those who have been affected by this dreadful event.

"My thoughts are also with the emergency services and everyone who is assisting in the rescue efforts."

NZ Quake Helplines:

• UK Foreign Office: 0044 207 008 1500
• Republic of Ireland Department of Foreign Affairs: 00353 1 478 0822