Hundreds of thousands of bogus malware-laden emails purportedly from Westpac were sent to Australians on Thursday, in what a security firm says is the fastest spreading email it can remember.
The malware emails were first identified at 9.30 on Thursday morning and security firm MailGuard said at 3.30pm it had blocked 318,000 going to clients' inboxes.
Earlier on Thursday only two of the 44 largest anti-virus applications were detecting the virus.
The emails have the subject "Westpac Secure Email Notification" and appear to come from "firstname.lastname@example.org". They instruct the recipient to open a malicious attachment.
MailGuard said its filters detected 1193 unique source IP addresses from countries including Australia, the US, Peru, Iran, Colombia and Mexico. It was likely they were a large "botnet" of infected computers.
Westpac said it was working with anti-virus companies to improve their detection of the malicious software and had notified customers of the "hoax".