A cyber threat in the form of an overseas based ransomware virus called 'CryptoLocker' forced Tauranga City Council IT staff to shut down all its computer systems yesterday.

TCC communications manager Aimee Driscoll explains the virus takes files on infected computers, encrypts them and then locks them.

'We had one come in. We were able to quickly isolate it, to ensure it didn't come into the business, we shut down our systems,” she says

Council computers systems were tested and brought back on line as they were found to be secure.

'Because our IT systems are so effective we were able to isolate it very quickly,” says Aimee. 'So our systems in terms of our alerts and security worked very well.”

The threat came from overseas, but because council IT systems were able to isolate it, no files were lost.

The cost to ratepayers of having council access limited to phones for a day is not known., says Aimee.

Wikipedia describes CryptoLocker as a ransomware trojan which targets computers running Microsoft Windows. It's believed to have first been posted to the Internet on September 5 2013.

It propagates via infected email attachments, when activated, the malware encrypts certain types of files stored on local and mounted network drives.

The malware then displays a message offering to decrypt the data if a ransom is paid. While the virus was easily removed, the files remained encrypted – until in May 2014 operation Tovar took down the botnet used to distribute the malware and a data base of encryption keys seized.