Jon Moffat, director of DoubleClick's Asia Pacific customer support, sent a bulletin to the company's clients on Wednesday to update them on the incident.
Moffat said the performance of the company's ad-servers was beginning to return to normal.
However, his bulletin gave few clues as to the reason for the attack.
"We are investigating the cause of the attack, which was similar in form to the attacks on others in our industry," said Moffat.
It's unclear to which other incidents Moffat referred. ZDNet Australia contacted DoubleClick's offices in Hong Kong and Australia for comment on the bulletin but the company declined the opportunity to be interviewed about the matter.
Search portal companies Google and Yahoo were targets of virus-related DoS attacks on Monday.
Moffat said DoubleClick would take steps to "resolve the situation permanently".
For more coverage on ZDNet Australia, click here.