The state's three major hospitals were without their computer systems for nearly 12 hours from 5.30am last Saturday.

Although patients' safety was not affected, staff at the Royal Hobart Hospital and the North-West Regional Hospital became so frustrated by the lack of computer services that they tried to make a code yellow staff crisis call.

However, they were unable to without computer communication.

It is understood that the Launceston General Hospital is the only hospital of the three with manual back-up for documents like patient records.

It managed to keep operating through the computer failure by calling in extra staff to manually retrieve the information needed.

The Health Services spokesman said that last weekend's breakdown was caused by a malfunction in the data centre management system which caused all four air conditioners to shut down.

``This resulted in a heat build-up in the room and the automated computer systems initiated a shutdown to protect data and equipment,'' he said.

The computer failure on January 1, which is understood to have also affected statewide services like Ambulance Tasmania, resulted from a smouldering wire, the spokesman said.

``It resulted in the smoke detection system switching off the power as designed to avoid a fire,'' he said.

He said that Ambulance Tasmania was not affected in the latest incident.

Australian Nursing Federation Tasmanian secretary Neroli Ellis said that the hospital computer system was almost brand new.

``It's a whole new integrated information system that people have waited for for years,'' she said.