The grim task of clearing hundreds of washed-up whale carcasses was under way in New Zealand Tuesday, with the beach closed over fears the bodies will fill with gas and explode as they decompose.

The sands at Farewell Spit where the nearly 700 pilot whales beached in one of the largest mass strandings ever in New Zealand were closed to the public after authorities declared the rotting bodies a health risk.

The Department of Conservation said the carcasses of more than 300 whales were being trucked to an inaccessible location where nature would be allowed to take its course.

Conservation spokesman Herb Christophers said dealing with so many dead pilot whales -- each of which can be six metres (20 feet) long -- was a logistical challenge.

"They're being moved but they haven't been cleared out yet, not by a long chalk," he told AFP.

"It's going to be a few days' work just getting them off the beach."

Before the whales were moved, conservation workers in bio-hazard suits punctured the carcasses in an attempt to stop them ballooning up with gas then popping.

"I've seen exploding whales, it's not a pretty sight," said Christophers.

Departmental ranger Amanda Harvey said many of the dead no longer resembled whales.

"Unfortunately, when a whale heats up, a lot of pressure builds up in their body and the only option is for them to explode," she told TVNZ.

In total, an estimated 666 whales were stranded in two pods on Friday and Saturday at Farewell Spit, on the northern tip of the South Island.

About three-quarters of the first pod of 416 died, while the survivors and the second pod of about 250 were successfully re-floated.

Christophers said no freshly beached whales had been found Tuesday, although wildlife officers remained on high alert because of the mammals' tendency to re-strand themselves.

Farewell Spit, about 150 kilometres (95 miles) west of the tourist town of Nelson, has witnessed at least nine pilot whale strandings in the past decade, although the latest was by far the largest.

The 26-kilometre (16-mile) hook of sand that protrudes out into the sea creates a shallow seabed that is believed to interfere with the whales' sonar navigation systems.