In an e-mail to customers, Internode said the problem -- which caused significant delivery delays and some duplications of customers' e-mail -- saw the rate of incoming e-mails peak at more than 50 times its usual level. It started Wednesday and lasted for around 100 hours.
But Internode's problem was nothing special, according to iiNet managing director Michael Malone, whose company had its own Thursday flood to the tune of 25 million spam e-mails.
"This is pretty routine stuff. Every ISP in the country has to deal with spam attacks on a regular basis, certainly every week," he said.
An OptusNet spokesperson said it saw more incoming e-mail on the weekend than it normally would, "but it was no higher than the weekday load" and did not affect customers.
A spokesperson from BigPond was not immediately available to comment on whether it also saw heightened levels of e-mail.
To permanently block the incoming flood, Internode said it had installed additional network hardware which consisted of a high-performance e-mail firewall.
"This new system is capable of rejecting incoming spam and virus attacks of this sort on a sustained basis and operates in addition to the anti-spam/anti-virus software," said Internode.
The ISP said it would do whatever it took to stem future spam deluges.