The Australian Taxation Office (ATO) has been mopping up, after a 'Melissa type' computer virus infected its mail network on Wednesday.
Detection of the virus prompted the ATO to close down its entire mail network - a combination of cc:Mail and Microsoft Outlook - to prevent the virus spreading.
"We haven't had to [close down the mail network] for many years, but the Melissa-type virus worries us because it spreads so rapidly," said John Growder, assistant commissioner of IT, ATO.
The virus was detected in a branch office, when staff noticed it was affecting mail, Growder said.
"We have reason to believe the virus was innocently introduced," he said. "We think it came in through some mail."
"Our [anti virus] software at the gateway didn't pick it up," he said.
Once the mail network had been closed down, IT staff at the ATO's 23 branch offices began running antivirus software across local servers in an effort to eradicate the virus.
End users were warned that the mail network had been shut down, and that they should avoid opening suspicious e-mail messages, via the ATO's internal intranet and internal broadcasts in individual ATO buildings.
Growder expected that all branch offices would be reconnected to the mail network by last night or this morning.
The ATO has added to its "arsenal of antivirus software" from "a number of sources" since Wednesday, Growder said, explaining that the ATO is running more than one antivirus package in an effort to combat the virus threat.
The Melissa virus, which uses a macro virus to replicate itself across networks, spread across networks globally in April 1999, propagating itself by sending itself to the first 50 names in a user's Microsoft Outlook address book.
Since Melissa was identified, many copy cat viruses have been detected.