Ihug customers have been warned to take extra care when opening emails received on April 27 and 28 as the company's anti-virus scanner wasn't functioning properly.
Marketing manager Duncan Shand says the Trend Micro virus filter signature updates were corrupted and weren't filtering as they were intended.
"That meant from about 11pm until six the next morning the virus scanner was letting stuff through."
The number of users online at that time were relatively low, says Shand, meaning only around 20,000 customers may have been affected.
"We sent emails out warning them to be careful just to be on the safe side."
So far Shand says the company has received no reports of infection.
"If anything it's been a quieter day than normal on our help desk".
Over the weekend, Telecom's ISP Xtra was hit with a massive email outage following a surge in the amount of traffic through its servers. Delays of more than a day were reported and it took several days for the backlog to be cleared. Telecom has been installing new server equipment and is tweaking its anti-spam and anti-virus filtering solutions to improve traffic flow.