Image caption The habitat of the Powelliphanta giant land snail is under threat. Picture: Darrell Grundy

Conservationists in New Zealand have blamed a technical glitch after 800 endangered giant land snails were accidentally frozen to death.

The rare snails, rescued from an area earmarked for coal mining, were kept in a temperature-controlled room run by the Department of Conservation (DoC).

However, a faulty gauge sent temperatures plunging below freezing.

Staff at the DoC's West Coast Conservancy in Hokitika are said to have been "very upset".

The Powelliphanta giant land snails were among 6,000 taken from the Stockton Plateau on South Island several years ago to make way for coal mining, New Zealand media reported.

About 4,000 of those have been relocated to new habitats.

Powelliphanta snails Carnivorous, eating mostly worms and sometimes slugs

Can measure up to 90mm (3.5in) across

Lay about 5-10 large eggs a year

Can live up to 20 years Source: New Zealand DoC

John Lyall, the conservancy's technical support manager, said a temperature probe in one of three containers had failed, making the room colder than the snails could stand.

He said the incident had been "very upsetting" for staff, who were committed to the care of the snails.

"We organised to have the probe replaced as soon as we noticed the problem," Mr Lyall told local media.

"And we've instituted a more regular set of monitoring checks and are currently organising to install an alert system."

Mr Lyall said 360 eggs had been hatched in the past year and they were confident the breeding programme would recover.

However, some environmentalists said the accident demonstrated what happened when development was allowed to displace wildlife from its natural habitat.

"First, their natural home was destroyed for a coalmine on Stockton Plateau, and now they've died in captivity," said Nicola Vallance of New Zealand's Forest and Bird organisation.

"This tragedy was entirely avoidable.

"Keeping our wildlife in fridges is obviously not how New Zealanders would like to care for native animals found nowhere else in the world.

"It's a sad fact that this has been the best option for them because moving them back to the wild in other parts of the West Coast has not worked," she added.