Rescuers in New Zealand on Wednesday completed a mass evacuation of tourists stranded after a devastating earthquake, as officials in the capital Wellington sealed off dozens of potentially dangerous office blocks.

The last of the trapped holidaymakers boarded the navy ship HMNZS Canterbury late Wednesday at Kaikoura, the South Island seaside town that bore the brunt of the powerful 7.8 tremor.

"We got them all out, it was a superhuman effort," Red Cross spokesman Simon Makker told AFP from the stricken town.

"The last of them were cheering and dancing down the street on their way out. It was pretty cool."

Vessels from the United States, Canada and Australia are also steaming toward the town to provide emergency supplies and logistical support.

At the time of the quake, Kaikoura's population of 2,000 was bolstered by about 1,000 tourists, attracted by the region's renowned whale-watching cruises.

They were stranded when the tremor, which claimed two lives, severed road and rail access, leaving the town short of water and with little power.

Makker said most of the visitors were airlifted out by military helicopters, with about 250 set to depart on the HMNZS Canterbury for nearby Christchurch late Wednesday.

"There might still be a few stragglers left, but the focus now turns to the welfare of the residents," he said.

The tremor was felt across the country, causing violent shaking in Wellington about 250 kilometres (155 miles) away.

The capital was initially thought to have escaped serious damage but the local council said Wednesday that engineering inspectors had raised concerns about some 60 downtown buildings.

At least one has been rated at imminent threat of collapse and will be carefully demolished, while another was a new government office block that was supposed to be quake-proof.

Prime Minister John Key said he was "staggered" at the building's failure given the strict building codes in Wellington, which sits on a network of fault lines.

"Questions need to be asked and that's going to happen," he told Radio New Zealand.

- International rescue -

The tremor, one of the most powerful ever in the quake-prone South Pacific nation, has been followed by more than 1,700 aftershocks, complicating relief efforts.

The quake triggered numerous landslides that dumped mountains of rocky debris on a main highway and ripped railway lines 10 metres (30 feet) off their track bed, while huge fissures opened up in roads and houses were rocked off their foundations.

New Zealand's official GeoNet said reconnaissance flights had noted between 80,0000 to 100,000 landslips.

The devastation on land forced rescuers to turn to air and sea resources.

The ships were due in Auckland this week for celebrations marking the New Zealand navy's 75th anniversary but instead diverted to the disaster zone.

"It's heartening to see overseas partners so willing to alter their plans and offer their assistance," Defence Minister Gerry Brownlee said, adding that the international vessels were Canada's HMC Vancouver, Australia's HMAS Darwin and the US destroyer USS Sampson.

A further three New Zealand warships are also involved and Wellington has accepted unspecified aid from Japan and Singapore.

The Sampson is the first US warship to visit New Zealand waters in 33 years, ending a ban sparked by a Cold War-era diplomatic spat over Wellington's ban on nuclear-powered vessels.

US Secretary of State John Kerry said on a visit to New Zealand on Sunday that its presence was a historic moment marking "the normalisation of our security cooperation".

The 7.8 quake is the joint second strongest ever recorded in New Zealand.

It stirred painful memories for residents of Christchurch, which was devastated five years ago by a 6.3 tremor that killed 185 people.

The country is on the boundary of the Australian and Pacific tectonic plates, which form part of the so-called "Ring of Fire", and experiences up to 15,000 tremors a year.