New Zealand paused for two minutes' silence on Wednesday to mark the first anniversary of the devastating Christchurch earthquake, which left 185 people dead.

At 12:51 pm (2351 GMT Tuesday), the moment the 6.3-magnitude earthquake hit New Zealand's second largest city, flattening office blocks and toppling buildings onto lunchtime crowds, the nation fell quiet to honour the dead.

About 60,000 people gathered for a solemn memorial at Christchurch's Hagley Park, where families of the victims locked arms and bowed their heads.

Some wept while others closed their eyes in prayer, with only the sound of a crying baby breaking the silence before 185 butterflies were released in a gesture organisers said symbolised departed souls and rebirth.

Ahead of the public memorial, Prime Minister John Key told a separate service for victims' families held near the shattered city centre that the earthquake was "one of our darkest days".

Key said the earthquake "wreaked havoc on an unimaginable scale", changing Christchurch forever.

"It twisted buildings, tore up roads, destroyed homes and shook us to the core," he told the multi-faith service. "Worst of all it stole 185 loved ones from us and injured so many more."

Key recalled visiting Christchurch's Latimer Square in the hours after the quake, as fires raged in collapsed buildings, choking dust filled the air, sirens blared and aftershocks continued to rattle the city.

"It was New Zealand, but not a New Zealand I've ever seen before... the earthquake took everyday life in Canterbury and tossed it on its head, but it could not break the spirit you are famous for," he said.

Key also acknowledged frustration among Christchurch residents at delays to a NZ$30 billion ($25 billion) rebuilding programme amid ongoing aftershocks, including major tremors in June and December which caused further damage.

"We have a long journey ahead of us," he said, reiterating the government's determination to rebuild the South Island city.

Underlining his words, a 2.9 magnitude aftershock, minor by Christchurch standards, struck about 10 kilometres (six miles) off the coast near Christchurch about 20 minutes after the ceremony concluded.

Key also paid tribute to emergency workers from New Zealand and overseas for their efforts in responding to the disaster.

"February 22 will forever be one of the darkest days in this proud nation's history," he said.

"It will also be a day when, at the worst of all times, the best of the human spirit was on display. That spirit is something no earthquake can take away."

At the Hagley Park service, Governor-General Jerry Mateparae read a message of condolence from Britain's Prince Charles and a video address was played from US Secretary of State Hillary Clinton, who visited the city just before the quake.

"Even those of us who were far away on that terrible day share your grief and we know it's been a struggle," she said.

Australian Prime Minister Julia Gillard also sent condolences, saying her country's sorrow and grief over the disaster had not diminished in the past year.

In Japan, a memorial service attended by about 150 people in black mourning attire was held at Toyama College of Foreign Languages, which lost 12 students in the quake.

"Even if we now cannot see you, 12 friends, you live in our mind," said quake survivor Narumi Kuroda.

Cleaning worker Murray Straight was among 140 people and organisations bestowed with heroism awards for their actions during the disaster.

Straight, who used a rickety ladder to rescue 15 people trapped on the first floor of a partially collapsed shop, said the constant aftershocks still rattling Christchurch had made life in the city an emotional rollercoaster.

"Every time we have some sort of a shake, a car goes past, and the ground rattles a bit, it's like another earthquake. Is it the start of a big one again? So it tests us all the time," he told AFP.