About 800 rare giant snails have been frozen to death after a technical glitch in a Department of Conservation cool room in Hokitika.

The snails were some of 6000 taken from the Stockton Plateau several years ago to make way for coal mining.

About 4000 snails have already been relocated to new habitats.

DOC public awareness officer Jose Watson said a temperature probe in one cool room, where the snails were being kept, failed at Labour Weekend.

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The probe sent the incorrect temperature to the cooler unit, and the cooler unit "went into overdrive'', she said.

"The first thing that was noticed was that the unit was going over-time.''

Ms Watson said DOC staff were unsure how long the snails had been exposed to the freezing temperatures - which they can withstand for short periods in the wild - in the cool room.

DOC had not announced the snail deaths sooner because it had had to assess how many had died.

"When we first looked we couldn't tell. We had to wait a couple of weeks before we could start assessing what was going on.''

Only a few snails in the cool room had survived, she said.

However, the relocation programme would not be too adversely affected because snail numbers would recover in a few years.

The snail captive programme costs about $125,000 a year, paid by state-owned coal producer Solid Energy.

Sadly for the snails, it is not the first time some of their number have died while in storage.

In 2006 one snail died while being kept in an icecream container in a fridge.

The death fuelled the long-running and bitter battle over relocation of the giant rare powelliphanta augustus snails to allow Solid Energy to access $400 million worth of high-quality coal at its West Coast Stockton mine near Westport.

DOC's West Coast conservancy technical support manager John Lyall said the snail deaths were very upsetting as staff were committed to their care.

Before the loss they had been proud to be celebrating a "great captive breeding result''.

The faulty temperature probe had been repaired and a new alert system would reduce the chance of recurrence. However, there was no guarantee other probes would not fail, Mr Lyall said.

"Fortunately we've already managed to relocate more than 60 per cent of the original population into new habitats and we still have more than 800 unaffected snails in the other cool rooms and environmental chambers.

"The remaining snails are breeding well, producing good numbers of eggs, and we expect the captive population to recover the loss within a few years.''