Rescuers in New Zealand on Tuesday began airlifting tourists stranded by a 7.8 earthquake that devastated parts of the South Island's rugged coast, as a navy ship headed to the stricken area to help.

Military helicopters started ferrying the first of 1,200 tourists trapped in the seaside town of Kaikoura, which bore the brunt of the quake that claimed two lives when it struck early Monday.

Officials said the US and Japanese militaries would also help the relief effort.

Huge landslides have cut Kaikoura's road and rail links, and police said water was running low, power was intermittent and hundreds of people were staying in evacuation shelters.

The town has a population of 2,000, which Prime Minister John Key said was bolstered by an extra 1,200 tourists, mostly international backpackers attracted by the area's popular whale-watching cruises.

Key said getting them out safely was top priority and four military helicopters started transporting them to nearby Christchurch with naval ship HMNZS Canterbury taking hundreds more when it arrives, likely Wednesday.

He estimated the quake repair bill would reach billions of dollars but the first job was delivering much-needed supplies to the town.

"It's more water and food, it's more chemical toilets, it's fixing up the road access, getting those tourists out and then ultimately the big clean-up job," he told TVNZ.

The Defence Force said a C-103 Hercules was on standby to drop supplies, while local media said up to 50 civilian helicopters were also being drafted into the evacuation effort.

Tourist Marie-Louise Forster said the quake's ferocity was like nothing she had ever experienced.

"We thought that someone was shaking our van and trying to get in," she told Fairfax New Zealand. "We were very afraid."

The tremor, one of the most powerful ever in the quake-prone South Pacific nation, hit just after midnight on Monday morning with more than 800 aftershocks complicating relief efforts.

The Canterbury warship set off from Auckland late Monday and its commanding officer Simon Rooke said it could take up to 500 people.

"We're going to pick them up by landing craft and sea boats and extract them to Lyttelton (in Christchurch) so they can get to a point of safety," he said.

Civil Defence Minister Gerry Brownlee said he had accepted a US offer to contribute two MH-60 helicopters and Japan would also provide assistance, although its exact nature was still under discussion.

Heavy rain and driving winds were hampering clean-up efforts, although life outside the main Kaikoura disaster zone was slowly returning to normal as roads opened and power was restored.

- 'Absolutely terrified' -

The quake triggered landslides that dumped mountains of rocky debris on a main highway and ripped railway tracks 10 metres (30 feet) off course.

Huge fissures opened up in roads and some houses were rocked off their foundations.

One person died at a historic homestead that collapsed at Kaikoura, with another killed at a remote property north of Christchurch.

Experts said the relatively low death toll was because the quake was centred on a sparsely populated area and hit at night, when people were in their homes.

It was felt across most of the country, causing severe shaking in the capital Wellington, about 250 kilometres (155 miles) away.

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

It also set off a tsunami alert, with seaside properties evacuated amid fears of five metre (16 feet) waves, which failed to eventuate.

Key flew over the quake's epicentre on Monday and said he was shocked to see such "utter devastation".

The New Zealand leader admitted he was concerned that tourism, the country's biggest export earner, would take a hit after images of the damage flashed around the world.

"People worry about earthquakes," he said, recounting visiting Hungarian President Janos Ader's experience of the latest tremor.

"He'd never been in an earthquake in his life, so he was in the James Cook Hotel (in Wellington) absolutely terrified at what was happening."

Key said New Zealand's strict construction codes meant its buildings were high quality and tourists' chances of being caught in a tremor were "truthfully very low".

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, mostly minor.